Apparatus and method for optimizing access to memory

ABSTRACT

A method and apparatus for optimizing access to memory, wherein the method includes the steps of receiving a first request for access to a memory, receiving at least two additional requests for access to the memory, and determining a first clock overhead associated with the first request for access to the memory. The method further includes the steps of determining an additional clock overhead associated with each of the at least two additional requests for access to the memory in conjunction with the first request, determining a combination of requests that can be processed together using an optimized overhead, and processing the combination of requests as a single request with the optimal overhead.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of Ser. No. 09/343,409 filedJun. 30, 1999, and claims benefit of Serial No. 60/095,972 filed Aug.10, 1998, and claims benefit of Serial No. 60/092,220 filed Jul. 8,1998, and a Provisional Patent Application Serial No. 60/144,097, filedon Jul. 16, 1999, U.S. Provisional Patent Application Serial No.60/144,098, filed on Jul. 16, 1999, U.S. Provisional Patent ApplicationSerial No. 60/144,283, filed on Jul. 16, 1999, U.S. Provisional PatentApplication Serial No. 60/144,286, filed on Jul. 16, 1999, U.S.Provisional Patent Application Serial No. 60/144,284, filed on Jul. 16,1999, and U.S. Provisional Patent Application Serial No. 60/144,094,filed on Jul. 16, 1999. The subject matter of these earlier filedapplications is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and apparatus for high performanceswitching in local area communications networks such as token ring,asynchronous transfer mode (ATM), ethernet, fast ethernet, and gigabitethernet environments, generally known as local area networks (LAN). Inparticular, the invention relates to a new switching architecture in anintegrated, modular, single chip solution, which can be implemented on asemiconductor substrate such as a silicon chip.

2. Description of the Related Art

As computer performance has increased in recent years, the demands oncomputer networks has significantly increased; faster computerprocessors and higher memory capabilities need networks with highbandwidth capabilities to enable high speed transfer of significantamounts of data. The well-known ethernet technology, which is based uponnumerous Institute of Electrical and Electronic Engineers (IEEE)ethernet standards, is one example of computer networking technologywhich has been able to be modified and improved to remain a viablecomputing technology. A more complete discussion of prior art networkingsystems can be found, for example, in SWITCHED AND FAST ETHERNET, byBreyer and Riley (Ziff-Davis, 1996), and numerous IEEE publicationsrelating to IEEE 802 standards. Based upon the Open Systems Interconnect(OSI) 7-layer reference model, network capabilities have grown throughthe development of repeaters, bridges, routers, and, more recently,“switches”, which operate with various types of communication media.Thickwire, thinwire, twisted pair, and optical fiber are examples ofmedia which has been used for computer networks. Switches, as theyrelate to computer networking and to ethernet, are hardware-baseddevices which control the flow of data packets or cells based upondestination address information which is available in each packet. Aproperly designed and implemented switch should be capable of receivinga packet and switching the packet to an appropriate output port at whatis referred to wirespeed or linespeed, which is the maximum speedcapability of the particular network. Basic ethernet wirespeed is up to10 megabits per second, and Fast Ethernet is up to 100 megabits persecond. The newest ethernet is referred to as gigabit ethernet, and iscapable of transmitting data over a network at a rate of up to 1,000megabits per second. As speed has increased, design constraints anddesign requirements have become more and more complex with respect tofollowing appropriate design and protocol rules and providing a lowcost, commercially viable solution. For example, high speed switchingrequires high speed memory to provide appropriate buffering of packetdata; conventional Dynamic Random Access Memory (DRAM) is relativelyslow, and requires hardware-driven refresh. The speed of DRAMs,therefore, as buffer memory in network switching, results in valuabletime being lost, and it becomes almost impossible to operate the switchor the network at linespeed. Furthermore, external central processingunit (CPU) involvement should be avoided, since CPU involvement alsomakes it almost impossible to operate the switch at linespeed.Additionally, as network switches have become more and more complicatedwith respect to requiring rules tables and memory control, a complexmulti-chip solution is necessary which requires logic circuitry,sometimes referred to as glue logic circuitry, to enable the variouschips to communicate with each other. Additionally, cost/benefittradeoffs are necessary with respect to expensive but fast static randomaccess memory (SRAM) versus inexpensive but slow DRAMs. Additionally,DRAMs, by virtue of their dynamic nature, require refreshing of thememory contents in order to prevent losses thereof. SRAMs do not sufferfrom the refresh requirement, and have reduced operational overheadwhich compared to DRAMs such as elimination of page misses, etc.Although DRAMs have adequate speed when accessing locations on the samepage, speed is reduced when other pages must be accessed.

Referring to the OSI 7-layer reference model discussed previously, andillustrated in FIG. 7, the higher layers typically have moreinformation. Various types of products are available for performingswitching-related functions at various levels of the OSI model. Hubs orrepeaters operate at layer one, and essentially copy and “broadcast”incoming data to a plurality of spokes of the hub. Layer twoswitching-related devices are typically referred to as multiportbridges, and are capable of bridging two separate networks. Bridges canbuild a table of forwarding rules based upon which MAC (media accesscontroller) addresses exist on which ports of the bridge, and passpackets which are destined for an address which is located on anopposite side of the bridge. Bridges typically utilize what is known asthe “spanning tree” algorithm to eliminate potential data loops; a dataloop is a situation wherein a packet endlessly loops in a networklooking for a particular address. The spanning tree algorithm defines aprotocol for preventing data loops. Layer three switches, sometimesreferred to as routers, can forward packets based upon the destinationnetwork address. Layer three switches are capable of learning addressesand maintaining tables thereof which correspond to port mappings.Processing speed for layer three switches can be improved by utilizingspecialized high performance hardware, and off loading the host CPU sothat instruction decisions do not delay packet forwarding.

SUMMARY OF THE INVENTION

The present invention is related to a method for optimizing access tomemory, wherein the method includes the steps of receiving a firstrequest for access to a memory, receiving at least two additionalrequests for access to the memory, and determining a first clockoverhead associated with the first request for access to the memory. Themethod further includes the steps of determining an additional clockoverhead associated with each of the at least two additional requestsfor access to the memory in conjunction with the first request,determining a combination of requests that can be processed togetherusing an optimized overhead, and processing the combination of requestsas a single request with the optimal overhead.

The present invention is further related to a method for optimizingaccess to sychronous dynamic random access memory (SDRAM) in a networkswitch, wherein the method includes the steps of receiving a pluralityof requests for access to an SDRAM, and combining at least two of theplurality of requests for processing as a single request utilizing anoptimal clock overhead, in accordance with a predetermined algorithm.

The present invention is further related to a method for optimizingSDRAM in a network switch including the steps of receiving a first,second, third, and fourth requests for access to the SDRAM. Determiningthe clock overhead associated with processing the first request inconjunction with the second request, determining a clock overheadassociated with processing the first request in conjunction with thethird request, and determining a clock overhead associated withprocessing the first request in conjunction with the fourth request.Finally, the method includes the steps of determining an optimal requestfor access to SDRAM, wherein the optimal request is calculated to yielda minimal clock overhead, and thereafter processing the optimal request.

The present invention is further related to an apparatus for optimizingaccess to memory in a network switch, wherein the apparatus includes ameans for receiving a first request for access to a memory, a means forreceiving at least two additional requests for access to the memory, anda means for determining a first clock overhead associated with the firstrequest for access to the memory. Further, the apparatus includes ameans for determining an additional clock overhead associated with eachof the at least two additional requests for access to the memory inconjunction with the first request, a means for determining acombination of requests that can be processed together using anoptimized overhead, and a means for processing the combination ofrequests as a single request with the optimal overhead.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention will be more readilyunderstood with reference to the following description and the attacheddrawings, wherein:

FIG. 1 is a general block diagram of elements of the present invention;

FIG. 2 is a more detailed block diagram of a network switch according tothe present invention;

FIG. 3 illustrates the data flow on the cell protocol sideband (CPS)channel of a network switch according to the present invention;

FIG. 4A illustrates demand priority round robin arbitration for accessto the C-channel of the network switch;

FIG. 4B illustrates access to the Cell-channel (C-channel) based uponthe round robin arbitration illustrated in FIG. 4A;

FIG. 5 illustrates Protocol-channel (P-channel) message types;

FIG. 6 illustrates a message format for Sideband-channel (S-channel)message types;

FIG. 7 is an illustration of the OSI 7 layer reference model;

FIG. 8 illustrates an operational diagram of an ethernet port interfacecontrollers (EPIC) module;

FIG. 9 illustrates the slicing of a data packet on the ingress to anEPIC module;

FIG. 10 is a detailed view of elements of the pipelined management unit(PMMU);

FIG. 11 illustrates the common buffer manager (CBM) cell format;

FIG. 12 illustrates an internal/external memory admission flow chart;

FIG. 13 illustrates a block diagram of an egress manager 76 illustratedin FIG. 10;

FIG. 14 illustrates more details of an EPIC module;

FIG. 15 is a block diagram of a fast filtering processor (FFP);

FIG. 16 is a block diagram of the elements of CPU management internetcontroller (CPIC) 40;

FIG. 17 illustrates a series of steps which are used to program an FFP;

FIG. 18 is a flow chart illustrating the aging process for addressresolution logic (ARL) (L2) and L3 tables;

FIG. 19 illustrates communication using a trunk group according to thepresent invention;

FIG. 20 is a detailed illustration of the Memory Management Unit (MMU);

FIG. 21 is a timing diagram for the MMU;

FIG. 22 is a timing diagram for the slot free address pool (SFAP) to theSDRAM Scheduler;

FIG. 23 is a timing diagram for the slot assembly unit (SAU) to theSDRAM Scheduler;

FIG. 24 is a timing diagram for the SDRAM Scheduler to the slotdisassembly unit (SDU);

FIG. 25 is a timing diagram for the SDRAM Controller interface;

FIG. 26 is a timing diagram for the SDRAM Controller DATA Writefirst-in-first-out (FIFO);

FIG. 27 is a timing diagram for the SDRAM Controller DATA Read FIFO;

FIG. 28 illustrates the first and second word formats;

FIG. 29 illustrates number of words within SAU and SDRAM that correspondto four possible two bit cell sizes;

FIG. 30 illustrates the SAU word format;

FIG. 31 illustrates a data storage configuration;

FIG. 32 illustrates the timing for the logical configuration shown inFIG. 31;

FIG. 33 is a flowchart for receiving a cell within status, location &budget manager (SLBM);

FIG. 34 is a flowchart local accrual process;

FIG. 35 is a flowchart of the global accrual process;

FIG. 36 is a flowchart of the continue local accrual process;

FIG. 37 is a flowchart of the continue global accrual process;

FIG. 38 is an illustration of flow control; and

FIG. 39 is a further illustration of the Memory Management Unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a configuration wherein a switch-on-chip (SOC) 10, inaccordance with the present invention, is functionally connected toexternal devices 11, external memory 12, fast ethernet ports 13, andgigabit ethernet ports 15. For the purposes of this embodiment, fastethernet ports 13 will be considered low speed ethernet ports, sincethey are capable of operating at speeds ranging from 10 Mbps to 100Mbps, while the gigabit ethernet ports 15, which are high speed ethernetports, are capable of operating at 1000 Mbps. External devices 11 couldinclude other switching devices for expanding switching capabilities, orother devices as may be required by a particular application. Externalmemory 12 is additional off-chip memory, which is in addition tointernal memory which is located on SOC 10, as will be discussed below.CPU 52 can be used as necessary to program SOC 10 with rules which areappropriate to control packet processing. However, once SOC 10 isappropriately programmed or configured, SOC 10 operates, as much aspossible, in a free running manner without communicating with CPU 52.Because CPU 52 does not control every aspect of the operation of SOC 10,CPU 52 performance requirements, at least with respect to SOC 10, arefairly low. A less powerful and therefore less expensive CPU 52 cantherefore be used when compared to known network switches. As also willbe discussed below, SOC 10 utilizes external memory 12 in an efficientmanner so that the cost and performance requirements of memory 12 can bereduced. Internal memory on SOC 10, as will be discussed below, is alsoconfigured to maximize switching throughput and minimize costs.

It should be noted that any number of fast ethernet ports 13 and gigabitethernet ports 15 can be provided. In one embodiment, a maximum of 24fast ethernet ports 13 and 2 gigabit ports 15 can be provided.Similarly, additional interconnect links to additional external devices11, external memory 12, and CPUs 52 may be provided as necessary.

FIG. 2 illustrates a more detailed block diagram of the functionalelements of SOC 10. As evident from FIG. 2 and as noted above, SOC 10includes a plurality of modular systems on-chip, with each modularsystem, although being on the same chip, being functionally separatefrom other modular systems. Therefore, each module can efficientlyoperate in parallel with other modules, and this configuration enables asignificant amount of freedom in updating and re-engineering SOC 10.

SOC 10 includes a plurality of Ethernet Port Interface Controllers(EPIC) 20 a, 20 b, 20 c, etc., a plurality of Gigabit Port InterfaceControllers (GPIC) 30 a, 30 b, etc., a CPU Management InterfaceController (CMIC) 40, a Common Buffer Memory Pool (CBP) 50, a PipelinedMemory Management Unit (PMMU) 70, including a Common Buffer Manager(CBM) 71, and a system-wide bus structure referred to as CPS channel 80.The PMMU 70 communicates with external memory 12, which includes aGlobal Buffer Memory Pool (GBP) 60. The CPS channel 80 comprises Cchannel 81, P channel 82, and S channel 83. The CPS channel is alsoreferred to as the Cell Protocol Sideband Channel, and is a 17 Gbpschannel which glues or interconnects the various modules together. Asalso illustrated in FIG. 2, other high speed interconnects can beprovided, as shown as an extendible high speed interconnect. In oneembodiment of the invention, this interconnect can be in the form of aninterconnect port interface controller (IPIC) 90, which is capable ofinterfacing CPS channel 80 to external devices 11 through an extendiblehigh speed interconnect link. As will be discussed below, each EPIC 20a, 20 b, and 20 c, generally referred to as EPIC 20, and GPIC 30 a and30 b, generally referred to as GPIC 30, are closely interrelated withappropriate address resolution logic and layer three switching tables 21a, 21 b, 21 c, 31 a, 31 b, rules tables 22 a, 22 b, 22 c, 31 a, 31 b,and virtual LAN (VLAN) tables 23 a, 23 b, 23 c, 31 a, 31 b. These tableswill be generally referred to as 21, 31, 22, 32, 23, 33, respectively.These tables, like other tables on SOC 10, are implemented in silicon astwo-dimensional arrays.

In a preferred embodiment of the invention, each EPIC 20 supports 8 fastethernet ports 13, and switches packets to and/or from these ports asmay be appropriate. The ports, therefore, are connected to the networkmedium (coaxial, twisted pair, fiber, etc.) using known media connectiontechnology, and communicates with the CPS channel 80 on the other sidethereof. The interface of each EPIC 20 to the network medium can beprovided through a Reduced Media Internal Interface (RMII), whichenables the direct medium connection to SOC 10. As is known in the art,auto-negotiation is an aspect of fast ethernet, wherein the network iscapable of negotiating a highest communication speed between a sourceand a destination based on the capabilities of the respective devices.The communication speed can vary, as noted previously, between 10 Mbpsand 100 Mbps; auto negotiation capability, therefore, is built directlyinto each EPIC module. The address resolution logic (ARL) and layerthree tables (ARL/L3) 21 a, 21 b, 21 c, rules table 22 a, 22 b, 22 c,and VLAN tables 23 a, 23 b, and 23 c are configured to be part of orinterface with the associated EPIC in an efficient and expedient manner,also to support wirespeed packet flow.

Each EPIC 20 has separate ingress and egress functions. On the ingressside, self-initiated and CPU-initiated learning of level 2 addressinformation can occur. Address resolution logic (ARL) is utilized toassist in this task. Address aging is built in as a feature, in order toeliminate the storage of address information which is no longer valid oruseful. The EPIC also carries out layer 2 mirroring. A fast filteringprocessor (FFP) 141 (see FIG. 14) is incorporated into the EPIC, inorder to accelerate packet forwarding and enhance packet flow. Theingress side of each EPIC and GPIC, illustrated in FIG. 8 as ingresssubmodule 14, has a significant amount of complexity to be able toproperly process a significant number of different types of packetswhich may come in to the port, for linespeed buffering and thenappropriate transfer to the egress. Functionally, each port on eachmodule of SOC 10 has a separate ingress submodule 14 associatedtherewith. From an implementation perspective, however, in order tominimize the amount of hardware implemented on the single-chip SOC 10,common hardware elements in the silicon will be used to implement aplurality of ingress submodules on each particular module. Theconfiguration of SOC 10 discussed herein enables concurrent lookups andfiltering, and therefore, processing of up to 6.6 million packets persecond. Layer two lookups, Layer three lookups and filtering occursimultaneously to achieve this level of performance. On the egress side,the EPIC is capable of supporting packet polling based either as anegress management or class of service (COS) function.Rerouting/scheduling of packets to be transmitted can occur, as well ashead-of-line (HOL) blocking notification, packet aging, cell reassembly,and other functions associated with ethernet port interface.

Each GPIC 30 is similar to each EPIC 20, but supports only one gigabitethernet port, and utilizes a port-specific ARL table, rather thanutilizing an ARL table which is shared with any other ports.Additionally, instead of an RMII, each GPIC port interfaces to thenetwork medium utilizing a gigabit media independent interface (GMII).

CMIC 40 acts as a gateway between the SOC 10 and the host CPU. Thecommunication can be, for example, along a peripheral componentinterconnect (PCI) bus, or other acceptable communications bus. CMIC 40can provide sequential direct mapped accesses between the host CPU 52and the SOC 10. CPU 52, through the CMIC 40, will be able to accessnumerous resources on SOC 10, including management information base(MIB) counters, programmable registers, status and control registers,configuration registers, ARL tables, port-based VLAN tables, IEEE 802.1qVLAN tables, layer three tables, rules tables, CBP address and datamemory, as well as GBP address and data memory. Optionally, the CMIC 40can include direct memory access (DMA) support, DMA chaining andscatter-gather, as Well as master and target PCI64.

Common buffer memory pool or CBP 50 can be considered to be the on-chipdata memory. In one embodiment of the invention, the CBP 50 is firstlevel high speed SRAM memory, to maximize performance and minimizehardware overhead requirements. The CBP can have a size of, for example,720 kilobytes running at 132 MHz. Packets stored in the CBP 50 aretypically stored as cells, rather than packets. As illustrated in thefigure, PMMU 70 also contains the Common Buffer Manager (CBM) 71thereupon. CBM 71 handles queue management, and is responsible forassigning cell pointers to incoming cells, as well as assigning commonpacket IDs (CPID) once the packet is fully written into the CBP. CBM 71can also handle management of the on-chip free address pointer pool,control actual data transfers to and from the data pool, and providememory budget management.

Global memory buffer pool or GBP 60 acts as a second level memory, andcan be located on-chip or off chip. In the preferred embodiment, GBP 60is located off chip with respect to SOC 10. When located off-chip, GBP60 is considered to be a part of or all of external memory 12. As asecond level memory, the GBP does not need to be expensive high speedSRAMs, and can be a slower less expensive memory such as DRAM. The GBPis tightly coupled to the PMMU 70, and operates like the CBP in thatpackets are stored as cells. For broadcast and multicast messages, onlyone copy of the packet is stored in GBP 60.

As shown in the figure, PMMU 70 is located between GBP 60 and CPSchannel 80, and acts as an external memory interface. In order tooptimize memory utilization, PMMU 70 includes multiple read and writebuffers, and supports numerous functions including global queuemanagement, which broadly includes assignment of cell pointers forrerouted incoming packets, maintenance of the global free address pool(FAP), time-optimized cell management, global memory budget management,GPID assignment and egress manager notification, write buffermanagement, read prefetches based upon egress manager/class of servicerequests, and smart memory control.

As shown in FIG. 2, the CPS channel 80 is actually three separatechannels, referred to as the C-channel, the P-channel, and theS-channel. The C-channel is 128 bits wide, and runs at 132 MHz. Packettransfers between ports occur on the C-channel. Since this channel isused solely for data transfer, there is no overhead associated with itsuse. The P-channel or protocol channel is synchronous or locked with theC-channel. During cell transfers, the message header is sent via theP-channel by the PMMU. The P-channel is 32 bits wide, and runs at 132MHz.

The S or sideband channel runs at 132 MHz, and is 32 bits wide. TheS-channel is used for functions such as four conveying Port Link Status,receive port full, port statistics, ARL table synchronization, memoryand register access to CPU and other CPU management functions, andglobal memory full and common memory full notification.

A proper understanding of the operation of SOC 10 requires a properunderstanding of the operation of CPS channel 80. Referring to FIG. 3,it can be seen that in SOC 10, on the ingress, packets are sliced by anEPIC 20 or GPIC 30 into 64-byte cells. The use of cells on-chip insteadof packets makes it easier to adapt the SOC to work with cell basedprotocols such as, for example, Asynchronous Transfer Mode (ATM).Presently, however, ATM utilizes cells which are 53 bytes long, with 48bytes for payload and 5 bytes for header. In the SOC, incoming packetsare sliced into cells which are 64 bytes long as discussed above, andthe cells are further divided into four separate 16 byte cell blocks Cn0. . . Cn3. Locked with the C-channel is the P-channel, which locks theopcode in synchronization with Cn0. A port bit map is inserted into theP-channel during the phase Cn1. The untagged bit map is inserted intothe P-channel during phase Cn2, and a time stamp is placed on theP-channel in Cn3. Independent from occurrences on the C and P-channel,the S-channel is used as a sideband, and is therefore decoupled fromactivities on the C and P-channel.

Cell or C-channel

Arbitration for the CPS channel occurs out of band. Every module (EPIC,GPIC, etc.) monitors the channel, and matching destination ports respondto appropriate transactions. C-channel arbitration is a demand priorityround robin arbitration mechanism. If no requests are active, however,the default module, which can be selected during the configuration ofSOC 10, can park on the channel and have complete access thereto. If allrequests are active, the configuration of SOC 10 is such that the PMMUis granted access every other cell cycle, and EPICs 20 and GPICs 30share equal access to the C-channel on a round robin basis. FIGS. 4A and4B illustrate a C-channel arbitration mechanism wherein section A is thePMMU, and section B consists of two GPICs and three EPICs. The sectionsalternate access, and since the PMMU is the only module in section A, itgains access every other cycle. The modules in section B, as notedpreviously, obtain access on a round robin basis.

Protocol or P-channel

Referring once again to the protocol or P-channel, a plurality ofmessages can be placed on the P-channel in order to properly direct flowof data flowing on the C-channel. Since P-channel 82 is 32 bits wide,and a message typically requires 128 bits, four smaller 32 bit messagesare put together in order to form a complete P-channel message. Thefollowing list identifies the fields and function and the various bitcounts of the 128 bit message on the P-channel.

Opcode—2 bits long—Identifies the type of message present on the Cchannel 81;

Internet Protocol (IP) Bit—1 bit long—This bit is set to indicate thatthe packet is an IP switched packet;

lnternetwork Packet Exchange (IPX) Bit—1 bit long—This bit is set toindicate that the packet is an IPX switched packet;

Next Cell—2 bits long—A series of values to identify the valid bytes inthe corresponding cell on the C channel 81;

Source Port Address Destination (SRC DEST Port)—6 bits long—Defines theport number which sends the message or receives the message, with theinterpretation of the source or destination depending upon Opcode;

Class of Service (COS or Cos)—3 bits long—Defines class of service forthe current packet being processed;

J—1 bit long—Describes whether the current packet is a jumbo packet;

S—1 bit long—Indicates whether the current cell is the first cell of thepacket;

E—1 bit long—Indicates whether the current cell is the last cell of thepacket;

CRC—2 bits long—Indicates whether a Cyclical Redundancy Check (CRC)value should be appended to the packet and whether a CRC value should beregenerated;

Bit—1 bit long—Determines whether MMU should Purge the entire packet;

Len—7 bytes—Identifies the valid number of bytes in current transfer;

O—2 bits—Defines an optimization for processing by the CPU 52; and

Broadcast/multicast (Bc/Mc) Bitmap—28 bits—Defines the broadcast ormulticast bitmap. Identifies egress ports to which the packet should beset, regarding multicast and broadcast messages.

Untag Bits/Source Port—28/5 bits long—Depending upon Opcode, the packetis transferred from Port to MMU, and this field is interpreted as theuntagged bit map. A different Opcode selection indicates that the packetis being transferred from MMU to egress port, and the last six bits ofthis field is interpreted as the Source Port field. The untagged bitsidentifies the egress ports which will strip the tag header, and thesource port bits identifies the port number upon which the packet hasentered the switch;

U Bit—1 bit long—For a particular Opcode selection (0x01, this bit beingset indicates that the packet should leave the port as Untagged; in thiscase, tag stripping is performed by the appropriate MAC;

CPU Opcode—18 bits long—These bits are set if the packet is being sentto the CPU for any reason. Opcodes are defined based upon filter match,learn bits being set, routing bits, destination lookup failure (DLF),station movement, etc;

Time Stamp—14 bits—The system puts a time stamp in this field when thepacket arrives, with a granularity of 1 μsec.

The opcode field of the P-channel message defines the type of messagecurrently being sent. While the opcode is currently shown as having awidth of 2 bits, the opcode field can be widened as desired to accountfor new types of messages as may be defined in the future. Graphically,however, the P-channel message type defined above is shown in FIG. 5.

An early termination message is used to indicate to CBM 71 that thecurrent packet is to be terminated. During operation, as discussed inmore detail below, the status bit (S) field in the message is set toindicate the desire to purge the current packet from memory. Also inresponse to the status bit all applicable egress ports would purge thecurrent packet prior to transmission.

The Src Dest Port field of the P-channel message, as stated above,define the destination and source port addresses, respectively. Eachfield is 6 bits wide and therefore allows for the addressing ofsixty-four ports.

The CRC field of the message is two bits wide and defines CRC actions.Bit 0 of the field provides an indication whether the associated egressport should append a CRC to the current packet. An egress port wouldappend a CRC to the current packet when bit 0 of the CRC field is set toa logical one. Bit 1 of the CRC field provides an indication whether theassociated egress port should regenerate a CRC for the current packet.An egress port would regenerate a CRC when bit 1 of the CRC field is setto a logical one. The CRC field is only valid for the last celltransmitted as defined by the E bit field of P-channel message set to alogical one.

As with the CRC field, the status bit field (st), the Len field, and theCell Count field of the message are only valid for the last cell of apacket being transmitted as defined by the E bit field of the message.

Last, the time stamp field of the message has a resolution of 1 μs andis valid only for the first cell of the packet defined by the S bitfield of the message. A cell is defined as the first cell of a receivedpacket when the S bit field of the message is set to a logical onevalue.

As is described in more detail below, the C channel 81 and the P channel82 are synchronously tied together such that data on C channel 81 istransmitted over the CPS channel 80 while a corresponding P channelmessage is simultaneously transmitted.

S-channel or Sideband Channel

The S channel 83 is a 32-bit wide channel which provides a separatecommunication path within the SOC 10. The S channel 83 is used formanagement by CPU 52, SOC 10 internal flow control, and SOC 10inter-module messaging. The S channel 83 is a sideband channel of theCPS channel 80, and is electrically and physically isolated from the Cchannel 81 and the P channel 82. It is important to note that since theS channel is separate and distinct from the C channel 81 and the Pchannel 82, operation of the S channel 83 can continue withoutperformance degradation related to the C channel 81 and P channel 82operation. Conversely, since the C channel is not used for thetransmission of system messages, but rather only data, there is nooverhead associated with the C channel 81 and, thus, the C channel 81 isable to free-run as needed to handle incoming and outgoing packetinformation.

The S channel 83 of CPS channel 80 provides a system wide communicationpath for transmitting system messages, for example, providing the CPU 52with access to the control structure of the SOC 10. System messagesinclude port status information, including port link status, receiveport full, and port statistics, ARL table 22 synchronization, CPU 52access to GBP 60 and CBP 50 memory buffers and SOC 10 control registers,and memory full notification corresponding to GBP 60 and/or CBP 50.

FIG. 6 illustrates a message format for an S channel message on Schannel 83. The message is formed of four 32-bit words; the bits of thefields of the words are defined as follows:

Opcode—6 bits long—Identifies the type of message present on the Schannel;

Dest Port—6 bits long—Defines the port number to which the current Schannel message is addressed;

Src Port—6 bits long—Defines the port number of which the current Schannel message originated;

COS—3 bits long—Defines the class of service associated with the currentS channel message; and

C bit—1 bit long—Logically defines whether the current S channel messageis intended for the CPU 52.

Error Code—2 bits long—Defines a valid error when the E bit is set;

DataLen—7 bits long—Defines the total number of data bytes in the Datafield;

E bit—1 bit long—Logically indicates whether an error has occurred inthe execution of the current command as defined by opcode;

Address—32 bits long—Defines the memory address associated with thecurrent command as defined in opcode;

Data—0-127 bits long—Contains the data associated with the currentopcode.

With the configuration of CPS channel 80 as explained above, thedecoupling of the S channel from the C channel and the P channel is suchthat the bandwidth on the C channel can be preserved for cell transfer,and that overloading of the C channel does not affect communications onthe sideband channel.

SOC Operation

The configuration of the SOC 10 supports fast ethernet ports, gigabitports, and extendible interconnect links as discussed above. The SOCconfiguration can also be “stacked”, thereby enabling significant portexpansion capability. Once data packets have been received by SOC 10,sliced into cells, and placed on CPS channel 80, stacked SOC modules caninterface with the CPS channel and monitor the channel, and extractappropriate information as necessary. As will be discussed below, asignificant amount of concurrent lookups and filtering occurs as thepacket comes in to ingress submodule 14 of an EPIC 20 or GPIC 30, withrespect to layer two and layer three lookups, and fast filtering.

Now referring to FIGS. 8 and 9, the handling of a data packet isdescribed. For explanation purposes, ethernet data to be received willconsider to arrive at one of the ports 24 a of EPIC 20 a. It will bepresumed that the packet is intended to be transmitted to a user on oneof ports 24 c of EPIC 20 c. All EPICs 20 (20 a, 20 b, 20 c, etc.) havesimilar features and functions, and each individually operate based onpacket flow.

An input data packet 112 is applied to the port 24 a is shown. The datapacket 112 is, in this example, defined per the current standards for10/100 Mbps Ethernet transmission and may have any length or structureas defined by that standard. This discussion will assume the length ofthe data packet 112 to be 1024 bits or 128 bytes.

When the data packet 112 is received by the EPIC module 20 a, an ingresssub-module 14 a, as an ingress function, determines the destination ofthe packet 112. The first 64 bytes of the data packet 112 is buffered bythe ingress sub-module 14 a and compared to data stored in the lookuptables 21 a to determine the destination port 24 c. Also as an ingressfunction, the ingress sub-module 14 a slices the data packet 112 into anumber of 64-byte cells; in this case, the 128 byte packet is sliced intwo 64 byte cells 112 a and 112 b. While the data packet 112 is shown inthis example to be exactly two 64-byte cells 112 a and 112 b, an actualincoming data packet may include any number of cells, with at least onecell of a length less than 64 bytes. Padding bytes are used to fill thecell. In such cases the ingress sub-module 14 a disregards the paddingbytes within the cell. Further discussions of packet handling will referto packet 112 and/or cells 112 a and 112 b.

It should be noted that each EPIC 20 (as well as each GPIC 30) has aningress submodule 14 and egress submodule 16, which provide portspecific ingress and egress functions. All incoming packet processingoccurs in ingress submodule 14, and features such as the fast filteringprocessor, layer two (L2) and layer three (L3) lookups, layer twolearning, both self-initiated and CPU 52 initiated, layer two tablemanagement, layer two switching, packet slicing, and channel dispatchingoccurs in ingress submodule 14. After lookups, fast filter processing,and slicing into cells, as noted above and as will be discussed below,the packet is placed from ingress submodule 14 into dispatch unit 18,and then placed onto CPS channel 80 and memory management is handled byPMMU 70. A number of ingress buffers are provided in dispatch unit 18 toensure proper handling of the packets/cells. Once the cells orcellularized packets are placed onto the CPS channel 80, the ingresssubmodule is finished with the packet. The ingress is not involved withdynamic memory allocation, or the specific path the cells will taketoward the destination. Egress submodule 16, illustrated in FIG. 8 assubmodule 16a of EPIC 20 a, monitors CPS channel 80 and continuouslylooks for cells destined for a port of that particular EPIC 20. When thePMMU 70 receives a signal that an egress associated with a destinationof a packet in memory is ready to receive cells, PMMU 70 pulls the cellsassociated with the packet out of the memory, as will be discussedbelow, and places the cells on CPS channel 80, destined for theappropriate egress submodule. A FIFO in the egress submodule 16continuously sends a signal onto the CPS channel 80 that it is ready toreceive packets, when there is room in the FIFO for packets or cells tobe received. As noted previously, the CPS channel 80 is configured tohandle cells, but cells of a particular packet are always handledtogether to avoid corrupting of packets. In order to overcome data flowdegradation problems associated with overhead usage of the C channel 81,all L2 learning and L2 table management is achieved through the use ofthe S channel 83. L2 self-initiated learning is achieved by decipheringthe source address of a user at a given ingress port 24 utilizing thepacket's associated address. Once the identity of the user at theingress port 24 is determined, the ARL/L3 tables 21 a are updated toreflect the user identification. The ARL/L3 tables 21 of each other EPIC20 and GPIC 30 are updated to reflect the newly acquired useridentification in a synchronizing step, as will be discussed below. As aresult, while the ingress of EPIC 20 a may determine that a given useris at a given port 24 a, the egress of EPIC 20 b, whose table 21 b hasbeen updated with the user's identification at port 24 a, can thenprovide information to the User at port 24 a without re-learning whichport the user was connected.

Table management may also be achieved through the use of the CPU 52. CPU52, via the CMIC 40, can provide the SOC 10 with software functionswhich result in the designation of the identification of a user at agiven port 24. As discussed above, it is undesirable for the CPU 52 toaccess the packet information in its entirety since this would lead toperformance degradation. Rather, the SOC 10 is programmed by the CPU 52with identification information concerning the user. The SOC 10 canmaintain real-time data flow since the table data communication betweenthe CPU 52 and the SOC 10 occurs exclusively on the S channel 83. Whilethe SOC 10 can provide the CPU 52 with direct packet information via theC channel 81, such a system setup is undesirable for the reasons setforth above. As stated above, as an ingress function an addressresolution lookup is performed by examining the ARL table 21 a. If thepacket is addressed to one of the layer three (L3) switches of the SOC10, then the ingress sub-module 14 a performs the L3 and default tablelookup. Once the destination port has been determined, the EPIC 20 asets a ready flag in the dispatch unit 18 a which then arbitrates for Cchannel 81.

The C channel 81 arbitration scheme, as discussed previously and asillustrated in FIGS. 4A and 4B, is Demand Priority Round-Robin. Eachinput/output (I/O) module, EPIC 20, GPIC 30, and CMIC 40, along with thePMMU 70, can initiate a request for C channel access. If no requestsexist at any one given time, a default module established with a highpriority gets complete access to the C channel 81. If any one single I/Omodule or the PMMU 70 requests C channel 81 access, that single modulegains access to the C channel 81 on-demand.

If EPIC modules 20 a, 20 b, 20 c, and GPIC modules 30 a and 30 b, andCMIC 40 simultaneously request C channel access, then access is grantedin round-robin fashion. For a given arbitration time period each of theI/O modules would be provided access to the C channel 81. For example,each GPIC module 30 a and 30 b would be granted access, followed by theEPIC modules, and finally the CMIC 40. After every arbitration timeperiod the next I/O module with a valid request would be given access tothe C channel 81. This pattern would continue as long as each of the I/Omodules provide an active C channel 81 access request.

If all the I/O modules, including the PMMU 70, request C channel 81access, the PMMU 70 is granted access as shown in FIG. 4B since the PMMUprovides a critical data path for all modules on the switch. Upongaining access to the channel 81, the dispatch unit 18 a proceeds inpassing the received packet 112, one cell at a time, to C channel 81.

Referring again to FIG. 3, the individual C, P, and S channels of theCPS channel 80 are shown. Once the dispatch unit 18 a has been givenpermission to access the CPS channel 80, during the first time periodCn0, the dispatch unit 18 a places the first 16 bytes of the first cell112 a of the received packet 112 on the C channel 81. Concurrently, thedispatch unit 18 a places the first P channel message corresponding tothe currently transmitted cell. As stated above, the first P channelmessage defines, among other things, the message type. Therefore, thisexample is such that the first P channel message would define thecurrent cell as being a unicast type message to be directed to thedestination egress port 21 c.

During the second clock cycle Cn1, the second 16 bytes (16:31) of thecurrently transmitted data cell 112 a are placed on the C channel 81.Likewise, during the second clock cycle Cn1, the Bc/Mc Port Bitmap isplaced on the P channel 82.

As indicated by the hatching of the S channel 83 data during the timeperiods Cn0 to Cn3 in FIG. 3, the operation of the S channel 83 isdecoupled from the operation of the C channel 81 and the P channel 82.For example, the CPU 52, via the CMIC 40, can pass system level messagesto non-active modules while an active module passes cells on the Cchannel 81. As previously stated, this is an important aspect of the SOC10 since the S channel operation allows parallel task processing,permitting the transmission of cell data on the C channel 81 inreal-time. Once the first cell 112 a of the incoming packet 112 isplaced on the CPS channel 80 the PMMU 70 determines whether the cell isto be transmitted to an egress port 21 local to the SOC 10.

If the PMMU 70 determines that the current cell 112 a on the C channel81 is destined for an egress port of the SOC 10, the PMMU 70 takescontrol of the cell data flow.

FIG. 10 illustrates, in more detail, the functional egress aspects ofPMMU 70. PMMU 70 includes CBM 71, and interfaces between the GBP, CBPand a plurality of egress managers (EgM) 76 of egress submodule 18, withone egress manager 76 being provided for each egress port. CBM 71 isconnected to each egress manager 76, in a parallel configuration, via Rchannel data bus 77. R channel data bus 77 is a 32-bit wide bus used byCBM 71 and egress managers 76 in the transmission of memory pointers andsystem messages. Each egress manager 76 is also connected to CPS channel80, for the transfer of data cells 112 a and 112 b.

CBM 71, in summary, performs the functions of on-chip FAP (free addresspool) management, transfer of cells to CBP 50, packet assembly andnotification to the respective egress managers, rerouting of packets toGBP 60 via a global buffer manager, as well as handling packet flow fromthe GBP 60 to CBP 50. Memory clean up, memory budget management, channelinterface, and cell pointer assignment are also functions of CBM 71.With respect to the free address pool, CBM 71 manages the free addresspool and assigns free cell pointers to incoming cells. The free addresspool is also written back by CBM 71, such that the released cellpointers from various egress managers 76 are appropriately cleared.Assuming that there is enough space available in CBP 50, and enough freeaddress pointers available, CBM 71 maintains at least two cell pointersper egress manager 76 which is being managed. The first cell of a packetarrives at an egress manager 76, and CBM 71 writes this cell to the CBMmemory allocation at the address pointed to by the first pointer. In thenext cell header field, the second pointer is written. The format of thecell as stored in CBP 50 is shown in FIG. 11; each line is 18 byteswide. Line 0 contains appropriate information with respect to first celland last cell information, broadcast/multicast, number of egress portsfor broadcast or multicast, cell length regarding the number of validbytes in the cell, the next cell pointer, total cell count in thepacket, and time stamp. The remaining lines contain cell data as 64 bytecells. The free address pool within PMMU 70 stores all free pointers forCBP 50. Each pointer in the free address pool points to a 64-byte cellin CBP 50; the actual cell stored in the CBP is a total of 72 bytes,with 64 bytes being byte data, and 8 bytes of control information.Functions such as HOL blocking high and low watermarks, out queue budgetregisters, CPID assignment, and other functions are handled in CBM 71,as explained herein.

When PMMU 70 determines that cell 112 a is destined for an appropriateegress port on SOC 10, PMMU 70 controls the cell flow from CPS channel80 to CBP 50. As the data packet 112 is received at PMMU 70 from CPS 80,CBM 71 determines whether or not sufficient memory is available in CBP50 for the data packet 112. A free address pool (not shown) can providestorage for at least two cell pointers per egress manager 76, per classof service. If sufficient memory is available in CBP 50 for storage andidentification of the incoming data packet, CBM 71 places the data cellinformation on CPS channel 80. The data cell information is provided byCBM 71 to CBP 50 at the assigned address. As new cells are received byPMMU 70, CBM 71 assigns cell pointers. The initial pointer for the firstcell 112 a points to the egress manager 76 which corresponds to theegress port to which the data packet 112 will be sent after it is placedin memory. In the example of FIG. 8, packets come in to port 24 a ofEPIC 20 a, and are destined for port 24 c of EPIC 20 c. For eachadditional cell 112 b, CBM 71 assigns a corresponding pointer. Thiscorresponding cell pointer is stored as a two byte or 16 bit valueNC_header, in an appropriate place on a control message, with theinitial pointer to the corresponding egress manager 76, and successivecell pointers as part of each cell header, a linked list of memorypointers is formed which defines packet 112 when the packet istransmitted via the appropriate egress port, in this case 24 c. Once thepacket is fully written into CBP 50, a corresponding CBP PacketIdentifier (CPID) is provided to the appropriate egress manager 76; thisCPID points to the memory location of initial cell 112 a. The CPID forthe data packet is then used when the data packet 112 is sent to thedestination egress port 24 c. In actuality, the CBM 71 maintains twobuffers containing a CBP cell pointer, with admission to the CBP beingbased upon a number of factors. An example of admission logic for CBP 50will be discussed below with reference to FIG. 12.

Since CBM 71 controls data flow within SOC 10, the data flow associatedwith any ingress port can likewise be controlled. When packet 112 hasbeen received and stored in CBP 50, a CPID is provided to the associatedegress manager 76. The total number of data cells associated with thedata packet is stored in a budget register (not shown). As more datapackets 112 are received and designated to be sent to the same egressmanager 76, the value of the budget register corresponding to theassociated egress manager 76 is incremented by the number of data cells112 a, 112 b of the new data cells received. The budget registertherefore dynamically represents the total number of cells designated tobe sent by any specific egress port on an EPIC 20. CBM 71 controls theinflow of additional data packets by comparing the budget register to ahigh watermark register value or a low watermark register value, for thesame egress.

When the value of the budget register exceeds the high watermark value,the associated ingress port is disabled. Similarly, when data cells ofan egress manager 76 are sent via the egress port, and the correspondingbudget register decreases to a value below the low watermark value, theingress port is once again enabled. When egress manager 76 initiates thetransmission of packet 112, egress manager 76 notifies CBM 71, whichthen decrements the budget register value by the number of data cellswhich are transmitted. The specific high watermark values and lowwatermark values can be programmed by the user via CPU 52. This givesthe user control over the data flow of any port on any EPIC 20 or GPIC30.

Egress manager 76 is also capable of controlling data flow. Each egressmanager 76 is provided with the capability to keep track of packetidentification information in a packet pointer budget register; as a newpointer is received by egress manager 76, the associated packet pointerbudget register is incremented. As egress manager 76 sends out a datapacket 112, the packet pointer budget register is decremented. When astorage limit assigned to the register is reached, corresponding to afull packet identification pool, a notification message is sent to allingress ports of the SOC 10, indicating that the destination egress portcontrolled by that egress manager 76 is unavailable. When the packetpointer budget register is decremented below the packet pool highwatermark value, a notification message is sent that the destinationegress port is now available. The notification messages are sent by CBM71 on the S channel 83.

As noted previously, flow control may be provided by CBM 71, and also byingress submodule 14 of either an EPIC 20 or GPIC 30. Ingress submodule14 monitors cell transmission into ingress port 24. When a data packet112 is received at an ingress port 24, the ingress submodule 14increments a received budget register by the cell count of the incomingdata packet. When a data packet 112 is sent, the corresponding ingress14 decrements the received budget register by the cell count of theoutgoing data packet 112. The budget register 72 is decremented byingress 14 in response to a decrement cell count message initiated byCBM 71, when a data packet 112 is successfully transmitted from CBP 50.

Efficient handling of the CBP and GBP is necessary in order to maximizethroughput, to prevent port starvation, and to prevent port underrun.For every ingress, there is a low watermark and a high watermark; ifcell count is below the low watermark, the packet is admitted to theCBP, thereby preventing port starvation by giving the port anappropriate share of CBP space.

FIG. 12 generally illustrates the handling of a data packet 112 when itis received at an appropriate ingress port. This figure illustratesdynamic memory allocation on a single port, and is applicable for eachingress port. In step 12-1, packet length is estimated by estimatingcell count based upon egress manager count plus incoming cell count.After this cell count is estimated, the GBP current cell count ischecked at step 12-2 to determine whether or not the GBP 60 is empty. Ifthe GBP cell count is 0, indicating that GBP 60 is empty, the methodproceeds to step 12-3, where it is determined whether or not theestimated cell count from step 12-1 is less than the admission lowwatermark. The admission low watermark value enables the reception ofnew packets 112 into CBP 50 if the total number of cells in theassociated egress is below the admission low watermark value. If yes,therefore, the packet is admitted at step 12-5. If the estimated cellcount is not below the admission low watermark, CBM 71 then arbitratesfor CBP memory allocation with other ingress ports of other EPICs andGPICs, in step 124. If the arbitration is unsuccessful, the incomingpacket is sent to a reroute process, referred to as A. If thearbitration is successful, then the packet is admitted to the CBP atstep 12-5. Admission to the CBP is necessary for linespeed communicationto occur.

The above discussion is directed to a situation wherein the GBP cellcount is determined to be 0. If in step 12-2 the GBP cell count isdetermined not to be 0, then the method proceeds to step 12-6, where theestimated cell count determined in step 12-1 is compared to theadmission high watermark. If the answer is no, the packet is rerouted toGBP 60 at step 12-7. If the answer is yes, the estimated cell count isthen compared to the admission low watermark at step 12-8. If the answeris no, which means that the estimated cell count is between the highwatermark and the low watermark, then the packet is rerouted to GBP 60at step 12-7. If the estimated cell count is below the admission lowwatermark, the GBP current count is compared With a reroute cell limitvalue at step 12-9. This reroute cell limit value is user programmablethrough CPU 52. If the GBP count is below or equal to the reroute celllimit value at step 12-9, the estimated cell count and GBP count arecompared with an estimated cell count low watermark; if the combinationof estimated cell count and GBP count are less than the estimated cellcount low watermark, the packet is admitted to the CBP. If the sum isgreater than the estimated cell count low watermark, then the packet isrerouted to GBP 60 at step 12-7. After rerouting to GBP 60, the GBP cellcount is updated, and the packet processing is finished. It should benoted that if both the CBP and the GBP are full, the packet is dropped.Dropped packets are handled in accordance with known ethernet or networkcommunication procedures, and have the effect of delaying communication.However, this configuration applies appropriate back pressure by settingwatermarks, through CPU 52, to appropriate buffer values on a per portbasis to maximize memory utilization. This CBP/GBP admission logicresults in a distributed hierarchical shared memory configuration, witha hierarchy between CBP 50 and GBP 60, and hierarchies within the CBP.

Address Resolution (L2)+(L3)

FIG. 14 illustrates some of the concurrent filtering and look-up detailsof a packet coming into the ingress side of an EPIC 20. FIG. 12, asdiscussed previously, illustrates the handling of a data packet withrespect to admission into the distributed hierarchical shared memory.FIG. 14 addresses the application of filtering, address resolution, andrules application segments of SOC 10. These functions are performedsimultaneously with respect to the CBP admission discussed above. Asshown in the figure, packet 112 is received at input port 24 of EPIC 20.It is then directed to input FIFO 142. As soon as the first sixteenbytes of the packet arrive in the input FIFO 142, an address resolutionrequest is sent to ARL engine 143; this initiates lookup in ARL/L3tables 21.

A description of the fields of an ARL table of ARL/L3 tables 21 is asfollows:

Mac Address—48 bits long—Mac Address;

VLAN tag—12 bits long—VLAN Tag Identifier as described in IEEE 802.1qstandard for tagged packets. For an untagged Packet, this value ispicked up from Port Based VLAN Table.

CosDst—3 bits long—Class of Service based on the Destination Address.COS identifies the priority of this packet. 8 levels of priorities asdescribed in IEEE 802.1p standard.

Port Number—6 bits long—Port Number is the port on which this Macaddress is learned.

SD_Disc Bits—2 bits long—These bits identifies whether the packet shouldbe discarded based on Source Address or Destination Address. Value 1means discard on source. Value 2 means discard on destination.

C bit—1 bit long—C Bit identifies that the packet should be given to CPUPort.

St Bit—1 bit long—St Bit identifies that this is a static entry (it isnot learned Dynamically) and that means is should not be aged out. OnlyCPU 52 can delete this entry.

Ht Bit—1 bit long—Hit Bit—This bit is set if there is match with theSource Address. It is used in the aging Mechanism.

CosSrc—3 bits long—Class of Service based on the Source Address. COSidentifies the priority of this packet.

L3 Bit—1 bit long—L3 Bit—identifies that this entry is created as resultof L3 Interface Configuration. The Mac address in this entry is L3interface Mac Address and that any Packet addresses to this Mac Addressneed to be routed.

T Bit—1 bit long—T Bit identifies that this Mac address is learned fromone of the Trunk Ports. If there is a match on Destination address thenoutput port is not decided on the Port Number in this entry, but isdecided by the Trunk Identification Process based on the rulesidentified by the RTAG bits and the Trunk group Identified by the TGID.

TGID—3 bits long—TGID identifies the Trunk Group if the T Bit is set.SOC 10 supports 6 Trunk Groups per switch.

RTAG—3 bits long—RTAG identifies the Trunk selection criterion if thedestination address matches this entry and the T bit is set in thatentry. Value 1—based on Source Mac Address. Value 2—based on DestinationMac Address. Value 3—based on Source & destination Address. Value4—based on Source IP Address. Value 5—based on Destination IP Address.Value 6—based on Source and Destination IP Address.

S C P—1 bit long—Source CoS Priority Bit—If this bit is set (in thematched Source Mac Entry) then Source CoS has priority over DestinationCos.

It should also be noted that VLAN tables 23 include a number of tableformats; all of the tables and table formats will not be discussed here.However, as an example, the port based VLAN table fields are describedas follows:

Port VLAN Id—12 bits long—Port VLAN Identifier is the VLAN Id used byPort Based VLAN.

Sp State—2 bits long—This field identifies the current Spanning TreeState. Value 0x00—Port is in Disable State. No packets are accepted inthis state, not even bridge protocol data unit (BPDU). Value 0x01—Portis in Blocking or Listening State. In this state no packets are acceptedby the port, except BPDUs. Value 0x02—Port is in Learning State. In thisstate the packets are not forwarded to another Port but are accepted forlearning. Value 0x03—Port is in Forwarding State. In this state thepackets are accepted both for learning and forwarding.

Port Discard Bits 6 bits long—There are 6 bits in this field and eachbit identifies the criterion to discard the packets coming in this port.Note: Bits 0 to 3 are not used. Bit 4—If this bit is set then all theframes coming on this port will be discarded. Bit 5—If this bit is setthen any 802.1q Priority Tagged (vid=0) and Untagged frame coming onthis port will be discarded.

J Bit—1 bit long—J Bit means Jumbo bit. If this bit is set then thisport should accept Jumbo Frames.

RTAG—3 bits long RTAG identifies the Trunk selection criterion if thedestination address matches this entry and the T bit is set in thatentry. Value 1—based on Source Mac Address. Value 2 based on DestinationMac Address. Value 3—based on Source & destination Address. Value4—based on Source IP Address. Value 5—based on Destination IP Address.Value 6—based on Source and Destination IP Address.

T Bit—1 bit long—This bit identifies that the Port is a member of theTrunk Group.

C Learn Bit—1 bit long—Cpu Learn Bit—If this bit is set then the packetis send to the CPU whenever the source Address is learned.

PT—2 bits long—Port Type identifies the port Type. Value 0—10 Mbit Port.Value 1—110 Mbit Port. Value 2—1 Gbit Port. Value 3—CPU Port.

VLAN Port Bitmap—28 bits long—VLAN Port Bitmap Identifies all the egressports on which the packet should go out.

B Bit—1 bit long—B bit is BPDU bit. If this bit is set then the Portrejects BPDUs. This Bit is set for Trunk Ports which are not supposed toaccept BPDUs.

TGID—3 bits long—TGID—this field identifies the Trunk Group which thisport belongs to.

Untagged Bitmap—28 bits long—This bitmap identifies the Untagged Membersof the VLAN. i.e. if the frame destined out of these members portsshould be transmitted without Tag Header.

M Bits—1 bit long—M Bit is used for Mirroring Functionality. If this bitis set then mirroring on Ingress is enabled.

The ARL engine 143 reads the packet; if the packet has a VLAN tagaccording to IEEE Standard 802.1q, then ARL engine 143 performs a lookupbased upon tagged VLAN table 231, which is part of VLAN table 23. If thepacket does not contain this tag, then the ARL engine performs VLANlookup based upon the port based VLAN table 232. Once the VLAN isidentified for the incoming packet, ARL engine 143 performs an ARL tablesearch based upon the source MAC address and the destination MACaddress. If the results of the destination search is an L3 interface MACaddress, then an L3 search, is performed of an L3 table within ARL/L3table 21. If the L3 search is successful, then the packet is modifiedaccording to packet routing rules. To better understand lookups,learning, and switching, it may be advisable to once again discuss thehandling of packet 112 with respect to FIG. 8. If data packet 112 issent from a source station A into port 24 a of EPIC 20 a, and destinedfor a destination station B on port 24 c of EPIC 20 c, ingress submodule14 a slices data packet 112 into cells 112 a and 112 b. The ingresssubmodule then reads the packet to determine the source MAC address andthe destination MAC address. As discussed previously, ingress submodule14 a, in particular ARL engine 143, performs the lookup of appropriatetables within ARL/L3 tables 21 a, and VLAN table 23 a, to see if thedestination MAC address exists in ARL/L3 tables 21 a; if the address isnot found, but if the VLAN IDs are the same for the source anddestination, then ingress submodule 14 a will set the packet to be sentto all ports. The packet will then propagate to the appropriatedestination address. A “source search” and a “destination search” occursin parallel. Concurrently, the source MAC address of the incoming packetis “learned”, and therefore added to an ARL table within ARL/L3 table 21a. After the packet is received by the destination, an acknowledgementis sent by destination station B to source station A. Since the sourceMAC address of the incoming packet is learned by the appropriate tableof B, the acknowledgement is appropriately sent to the port on which Ais located. When the acknowledgement is received at port 24 a,therefore, the ARL table learns the source MAC address of B from theacknowledgement packet. It should be noted that as long as the VLAN IDs(for tagged packets) of source MAC addresses and destination MACaddresses are the same, layer two switching as discussed above isperformed. L2 switching and lookup is therefore based on the first 16bytes of an incoming packet. For untagged packets, the port number fieldin the packet is indexed to the port-based VLAN table within VLAN table23 a, and the VLAN ID can then be determined. If the VLAN IDs aredifferent, however, L3 switching is necessary wherein the packets aresent to a different VLAN. L3 switching, however, is based on the IPheader field of the packet. The IP header includes source IP address,destination IP address, and TTL (time-to-live).

In order to more clearly understand layer three switching according tothe invention, data packet 112 is sent from source station A onto port24 a of EPIC 20 a, and is directed to destination station B; assume,however, that station B is disposed on a different VLAN, as evidenced bythe source MAC address and the destination MAC address having differingVLAN IDs. The lookup for B would be unsuccessful since B is located on adifferent VLAN, and merely sending the packet to all ports on the VLANwould result in B never receiving the packet. Layer three switching,therefore, enables the bridging of VLAN boundaries, but requires readingof more packet information than just the MAC addresses of L2 switching.In addition to reading the source and destination MAC addresses,therefore, ingress 14 a also reads the IP address of the source anddestination. As noted previously, packet types are defined by IEEE andother standards, and are known in the art. By reading the IP address ofthe destination, SOC 10 is able to target the packet to an appropriaterouter interface which is consistent with the destination IP address.Packet 112 is therefore sent on to CPS channel 80 through dispatch unit18 a, destined for an appropriate router interface (not shown, and notpart of SOC 10), upon which destination B is located. Control frames,identified as such by their destination address, are sent to CPU 52 viaCMIC 40. The destination MAC address, therefore, is the router MACaddress for B. The router MAC address is learned through the assistanceof CPU 52, which uses an ARP (address resolution protocol) request torequest the destination MAC address for the router for B, based upon theIP address of B. Through the use of the IP address, therefore, SOC 10can learn the MAC address. Through the acknowledgement and learningprocess, however, it is only the first packet that is subject to this“slow” handling because of the involvement of CPU 52. After theappropriate MAC addresses are learned, linespeed switching can occurthrough the use of concurrent table lookups since the necessaryinformation will be learned by the tables. Implementing the tables insilicon as two-dimensional arrays enables such rapid concurrent lookups.Once the MAC address for B has been learned, therefore, when packetscome in with the IP address for B, ingress 14 a changes the IP addressto the destination MAC address, in order to enable linespeed switching.Also, the source address of the incoming packet is changed to the routerMAC address for A rather than the IP address for A, so that theacknowledgement from B to A can be handled in a fast manner withoutneeding to utilize a CPU on the destination end in order to identify thesource MAC address to be the destination for the acknowledgement.Additionally, a TTL (time-to-live) field in the packet is appropriatelymanipulated in accordance with the IETF (Internet Engineering TaskForce) standard. A unique aspect of SOC 10 is that all of the switching,packet processing, and table lookups are performed in hardware, ratherthan requiring CPU 52 or another CPU to spend time processinginstructions. It should be noted that the layer three tables for EPIC 20can have varying sizes; in a preferred embodiment, these tables arecapable of holding up to 2000 addresses, and are subject to purging anddeletion of aged addresses, as explained herein.

Referring again to the discussion of FIG. 14, as soon as the first 64(sixty four) bytes of the packet arrive in input FIFO 142, a filteringrequest is sent to FFP 141 FFP 141 is an extensive filtering mechanismwhich enables SOC 10 to set inclusive and exclusive filters on any fieldof a packet from layer 2 to layer 7 of the OSI seven layer model.Filters are used for packet classification based upon a protocol fieldsin the packets. Various actions are taken based upon the packetclassification, including packet discard, sending of the packet to theCPU, sending of the packet to other ports, sending the packet on certainCOS priority queues, changing the type of service (TOS) precedence. Theexclusive filter is primarily used for implementing security features,and allows a packet to proceed only if there is a filter match. If thereis no match, the packet is discarded.

It should be noted that SOC 10 has a unique capability to handle bothtagged and untagged packets coming in. Tagged packets are tagged inaccordance with IEEE standards, and include a specific IEEE 802.1ppriority field for the packet. Untagged packets, however, do not includean 802.1p priority field therein. SOC 10 can assign an appropriate COSvalue for the packet, which can be considered to be equivalent to aweighted priority, based either upon the destination address or thesource address of the packet, as matched in one of the table lookups. Asnoted in the ARL table format discussed herein, an SCP (Source COSPriority) bit is contained as one of the fields of the table. When thisSCP bit is set, then SOC 10 will assign weighted priority based upon asource COS value in the ARL table. If the SCP is not set, then SOC 10will assign a COS for the packet based upon the destination COS field inthe ARL table. These COS values are three bit fields in the ARL table,as noted previously in the ARL table field descriptions.

FFP 141 is essentially a state machine driven programmable rules engine.The filters used by the FFP are 64 (sixty-four) bytes wide, and areapplied on an incoming packet; any offset can be used, however, apreferred embodiment uses an offset of zero, and therefore operates onthe first 64 bytes, or 512 bits, of a packet. The actions taken by thefilter are tag insertion, priority mapping, TOS tag insertion, sendingof the packet to the CPU, dropping of the packet, forwarding of thepacket to an egress port, and sending the packet to a mirrored port. Thefilters utilized by FFP 141 are defined by rules table 22. Rules table22 is completely programmable by CPU 52, through CMIC 40. The rulestable can be, for example, 256 entries deep, and may be partitioned forinclusive and exclusive filters, with, again as an example, 128 entriesfor inclusive filters and 128 entries for exclusive filters. A filterdatabase, within FFP 141, includes a number of inclusive mask registersand exclusive mask registers, such that the filters are formed basedupon the rules in rules table 22, and the filters therefore essentiallyform a 64 byte wide mask or bit map which is applied on the incomingpacket. If the filter is designated as an exclusive filter, the filterwill exclude all packets unless there is a match. In other words, theexclusive filter allows a packet to go through the forwarding processonly if there is a filter match. If there is no filter match, the packetis dropped. In an inclusive filter, if there is no match, no action istaken but the packet is not dropped. Action on an exclusive filterrequires an exact match of all filter fields. If there is an exact matchwith an exclusive filter, therefore, action is taken as specified in theaction field; the actions which may be taken, are discussed above. Ifthere is no full match or exact of all of the filter fields, but thereis a partial match, then the packet is dropped. A partial match isdefined as either a match on the ingress field, egress field, or filterselect fields. If there is neither a full match nor a partial match withthe packet and the exclusive filter, then no action is taken and thepacket proceeds through the forwarding process. The FFP configuration,taking action based upon the first 64 bytes of a packet, enhances thehandling of real time traffic since packets can be filtered and actioncan be taken on the fly. Without an FFP according to the invention, thepacket would need to be transferred to the CPU for appropriate action tobe interpreted and taken. For inclusive filters, if there is a filtermatch, action is taken, and if there is no filter match, no action istaken; however, packets are not dropped based on a match or no matchsituation for inclusive filters.

In summary, the FFP includes a filter database with eight sets ofinclusive filters and eight sets of exclusive filters, as separatefilter masks. As a packet comes into the FFP, the filter masks areapplied to the packet; in other words, a logical AND operation isperformed with the mask and the packet. If there is a match, thematching entries are applied to rules tables 22, in order to determinewhich specific actions will be taken. As mentioned previously, theactions include 802.1p tag insertion, 802.1p priority mapping, IP TOS(type-of-service) tag insertion, sending of the packet to the CPU,discarding or dropping of the packet, forwarding the packet to an egressport, and sending the packet to the mirrored port. Since there are alimited number of fields in the rules table, and since particular rulesmust be applied for various types of packets, the rules tablerequirements are minimized in the present invention by the presentinvention setting all incoming packets to be “tagged” packets; alluntagged packets, therefore, are subject to 802.1p tag insertion, inorder to reduce the number of entries which are necessary in the rulestable. This action eliminates the need for entries regarding handling ofuntagged packets. It should be noted that specific packet types aredefined by various IEEE and other networking standards, and will not bedefined herein.

As noted previously, exclusive filters are defined in the rules table asfilters which exclude packets for which there is no match; excludedpackets are dropped. With inclusive filters, however, packets are notdropped in any circumstances. If there is a match, action is taken asdiscussed above; if there is no match, no action is taken and the packetproceeds through the forwarding process. Referring to FIG. 15, FFP 141is shown to include filter database 1410 containing filter maskstherein, communicating with logic circuitry 1411 for determining packettypes and applying appropriate filter masks. After the filter mask isapplied as noted above, the result of the application is applied torules table 22, for appropriate lookup and action. It should be notedthat the filter masks, rules tables, and logic, while programmable byCPU 52, do not rely upon CPU 52 for the processing and calculationthereof. After programming, a hardware configuration is provided whichenables linespeed filter application and lookup.

Referring once again to FIG. 14, after FFP 141 applies appropriateconfigured filters and results are obtained from the appropriate rulestable 22, logic 1411 in FFP 141 determines and takes the appropriateaction. The filtering logic can discard the packet, send the packet tothe CPU 52, modify the packet header or IP header, and recalculate anyIP checksum fields or takes other appropriate action with respect to theheaders. The modification occurs at buffer slicer 144, and the packet isplaced on C channel 81. The control message and message headerinformation is applied by the FFP 141 and ARL engine 143, and themessage header is placed on P channel 82. Dispatch unit 18, alsogenerally discussed with respect to FIG. 8, coordinates all dispatchesto C channel, P channel and S channel. As noted previously, each EPICmodule 20, GPIC module 30, PMMU 70, etc. are individually configured tocommunicate via the CPS channel. Each module can be independentlymodified, and as long as the CPS channel interfaces are maintained,internal modifications to any modules such as EPIC 20 a should notaffect any other modules such as EPIC 20 b, or any GPICs 30.

As mentioned previously, FFP 141 is programmed by the user, through CPU52, based upon the specific functions which are sought to be handled byeach FFP 141. Referring to FIG. 17, it can be seen that in step 17-1, anFFP programming step is initiated by the user. Once programming has beeninitiated, the user identifies the protocol fields of the packet whichare to be of interest for the filter, in step 17-2. In step 17-3, thepacket type and filter conditions are determined, and in step 17-4, afilter mask is constructed based upon the identified packet type, andthe desired filter conditions. The filter mask is essentially a bit mapwhich is applied or ANDed with selected fields of the packet. After thefilter mask is constructed, it is then determined whether the filterwill be an inclusive or exclusive filter, depending upon the problemswhich are sought to be solved, the packets which are sought to beforwarded, actions sought to be taken, etc. In step 17-6, it isdetermined whether or not the filter is on the ingress port, and in step17-7, it is determined whether or not the filter is on the egress port.If the filter is on the ingress port, an ingress port mask is used instep 17-8. If it is determined that the filter will be on the egressport, then an egress mask is used in step 17-9. Based upon these steps,a rules table entry for rules tables 22 is then constructed, and theentry or entries are placed into the appropriate rules table (steps17-10 and 17-11). These steps are taken through the user inputtingparticular sets of rules and information into CPU 52 by an appropriateinput device, and CPU 52 taking the appropriate action with respect tocreating the filters, through CMIC 40 and the appropriate ingress oregress submodules on an appropriate EPIC module 20 or GPIC module 30.

It should also be noted that the block diagram of SOC 10 in FIG. 2illustrates each GPIC 30 having its own ARL/L3 tables 31, rules table32, and VLAN tables 33, and also each EPIC 20 also having its own ARL/L3tables 21, rules table 22, and VLAN tables 23. In a preferred embodimentof the invention, however, two separate modules can share a commonARL/L3 table and a common VLAN table. Each module, however, has its ownrules table 22. For example, therefore, GPIC 30 a may share ARL/L3 table21 a and VLAN table 23 a with EPIC 20 a. Similarly, GPIC 30 b may shareARL table 21 b and VLAN table 23 b with EPIC 20 b. This sharing oftables reduces the number of gates which are required to implement theinvention, and makes for simplified lookup and synchronization as willbe discussed below.

Table Synchronization and Aging

SOC 10 utilizes a unique method of table synchronization and aging, toensure that only current and active address information is maintained inthe tables. When ARL/L3 tables are updated to include a new sourceaddress, a “hit bit” is set within the table of the “owner” or obtainingmodule to indicate that the address has been accessed. Also, when a newaddress is learned and placed in the ARL table, an S channel message isplaced on S channel 83 as an ARL insert message, instructing all ARL/L3tables on SOC 10 to learn this new address. The entry in the ARL/L3tables includes an identification of the port which initially receivedthe packet and learned the address. Therefore, if EPIC 20 a contains theport which initially received the packet and therefore which initiallylearned the address, EPIC 20 a becomes the “owner” of the address. OnlyEPIC 20 a, therefore, can delete this address from the table. The ARLinsert message is received by all of the modules, and the address isadded into all of the ARL/L3 tables on SOC 10. CMIC 40 will also sendthe address information to CPU 52. When each module receives and learnsthe address information, an acknowledge (ACK) message is sent back toEPIC 20 a; as the owner further ARL insert messages cannot be sent fromEPIC 20 a until all ACK messages have been received from all of themodules. In a preferred embodiment of the invention, CMIC 40 does notsend an ACK message, since CMIC 40 does not include ingress/egressmodules thereupon, but only communicates with CPU 52. If multiple SOC 10are provided in a stacked configuration, all ARL/L3 tables would besynchronized due to the fact that CPS channel 80 would be sharedthroughout the stacked modules.

Referring to FIG. 18, the ARL aging process is discussed. An age timeris provided within each EPIC module 20 and GPIC module 30, at step 18-1,it is determined whether the age timer has expired. If the timer hasexpired, the aging process begins by examining the first entry in ARLtable 21. At step 18-2, it is determined whether or not the portreferred to in the ARL entry belongs to the particular module. If theanswer is no, the process proceeds to step 18-3, where it is determinedwhether or not this entry is the last entry in the table. If the answeris yes at step 18-3, the age timer is restarted and the process iscompleted at step 18-4. If this is not the last entry in the table, thenthe process is returned to the next ARL entry at step 18-5. If, however,at step 18-2 it is determined that the port does belong to thisparticular module, then, at step 18-6 it is determined whether or notthe hit bit is set, or if this is a static entry. If the hit bit is set,the hit bit is reset at step 18-7, and the method then proceeds to step18-3. If the hit bit is not set, the ARL entry is deleted at step 18-8,and a delete ARL entry message is sent on the CPS channel to the othermodules, including CMIC 40, so that the table can be appropriatelysynchronized as noted above. This aging process can be performed on theARL (layer two) entries, as well as layer three entries, in order toensure that aged packets are appropriately deleted from the tables bythe owners of the entries. As noted previously, the aging process isonly performed on entries where the port referred to belongs to theparticular module which is performing the aging process. To this end,therefore, the hit bit is only set in the owner module. The hit bit isnot set for entries in tables of other modules which receive the ARLinsert message. The hit bit is therefore always set to zero in thesynchronized non-owner tables.

The purpose of the source and destination searches, and the overalllookups, is to identify the port number within SOC 10 to which thepacket should be directed to after it is placed either CBP 50 or GBP 60.Of course, a source lookup failure results in learning of the sourcefrom the source MAC address information in the packet; a destinationlookup failure, however, since no port would be identified, results inthe packet being sent to all ports on SOC 10. As long as the destinationVLAN ID is the same as the source VLAN ID, the packet will propagate theVLAN and reach the ultimate destination, at which point anacknowledgement packet will be received, thereby enabling the ARL tableto learn the destination port for use on subsequent packets. If the VLANIDs are different, an L3 lookup and learning process will be performed,as discussed previously. It should be noted that each EPIC and each GPICcontains a FIFO queue to store ARL insert messages, since, although eachmodule can only send one message at a time, if each module sends aninsert message, a queue must be provided for appropriate handling of themessages.

Port Movement

After the ARL/L3 tables have entries in them, the situation sometimesarises where a particular user or station may change location from oneport to another port. In order to prevent transmission errors,therefore, SOC 10 includes capabilities of identifying such movement,and updating the table entries appropriately. For example, if station A,located for example on port 1, seeks to communicate with station B,whose entries indicate that user B is located on port 26. If station Bis then moved to a different port, for example, port 15, a destinationlookup failure will occur and the packet will be sent to all ports. Whenthe packet is received by station B at port 15, station B will send anacknowledge (ACK) message, which will be received by the ingress of theEPIC/GPIC module containing port 1 thereupon. A source lookup (of theacknowledge message) will yield a match on the source address, but theport information will not match. The EPIC/GPIC which receives the packetfrom B, therefore, must delete the old entry from the ARL/L3 table, andalso send an ARL/L3 delete message onto the S channel so that all tablesare synchronized. Then, the new source information, with the correctport, is inserted into the ARL/L3 table, and an ARL/L3 insert message isplaced on the S channel, thereby synchronizing the ARL/L3 tables withthe new information. The updated ARL insert message cannot be sent untilall of the acknowledgement messages are sent regarding the ARL deletemessage, to ensure proper table synchronization. As stated previously,typical ARL insertion and deletion commands can only be initiated by theowner module. In the case of port movement, however, since port movementmay be identified by any module sending a packet to a moved port, theport movement-related deletion and insertion messages can be initiatedby any module.

Trunking

During the configuration process wherein a local area network isconfigured by an administrator with a plurality of switches, etc.,numerous ports can be “trunked” to increase bandwidth. For example, iftraffic between a first switch (SW1) and a second switch (SW2) isanticipated as being high, the LAN can be configured such that aplurality of ports, for example ports 1 and 2, can be connectedtogether. In a 100 megabits per second environment, the trunking of twoports effectively provides an increased bandwidth of 200 megabits persecond between the two ports. The two ports 1 and 2, are thereforeidentified as a trunk group, and CPU 52 is used to properly configurethe handling of the trunk group. Once a trunk group is identified, it istreated as a plurality of ports acting as one logical port. FIG. 19illustrates a configuration wherein SW1, containing a plurality of portsthereon, has a trunk group with ports 1 and 2 of SW2, with the trunkgroup being two communication lines connecting ports 1 and 2 of each ofSW1 and SW2. This forms trunk group T. In this example, station A,connected to port 3 of SW1, is seeking to communicate or send a packetto station B, located on port 26 of switch SW2. The packet must travel,therefore, through trunk group T from port 3 of SW1 to port 26 of SW2.It should be noted that the trunk group could include any of a number ofports between the switches. As traffic flow increases between SW1 andSW2, trunk group T could be reconfigured by the administrator to includemore ports, thereby effectively increasing bandwidth. In addition toproviding increased bandwidth, trunking provides redundancy in the eventof a failure of one of the links between the switches. Once the trunkgroup is created, a user programs SOC 10 through CPU 52 to recognize theappropriate trunk group or trunk groups, with trunk group identification(TGID) information. A trunk group port bit map is prepared for eachTGID; and a trunk group table, provided for each module on SOC 10, isused to implement the trunk group, which can also be called a portbundle. A trunk group bit map table is also provided. These two tablesare provided on a per module basis, and, like tables 21, 22, and 23, areimplemented in silicon as two-dimensional arrays. In one embodiment ofSOC 10, six trunk groups can be supported, with each trunk group havingup to eight trunk ports thereupon. For communication, however, in orderto prevent out-of-ordering of packets or frames, the same port must beused for packet flow. Identification of which port will be used forcommunication is based upon any of the following: source MAC address,destination MAC address, source IP address, destination IP address, orcombinations of source and destination addresses. If source MAC is used,as an example, if station A on port 3 of SW1 is seeking to send a packetto station B on port 26 of SW2, then the last three bits of the sourceMAC address of station A, which are in the source address field of the apacket, are used to generate a trunk port index. The trunk port index,which is then looked up on the trunk group table by the ingresssubmodule 14 of the particular port on the switch, in order to determinewhich port of the trunk group will be used for the communication. Inother words, when a packet is sought to be sent from station A tostation B, address resolution is conducted as set forth above. If thepacket is to be handled through a trunk group, then a T bit will be setin the ARL entry which is matched by the destination address. If the Tbit or trunk bit is set, then the destination address is learned fromone of the trunk ports. The egress port, therefore, is not learned fromthe port number obtained in the ARL entry, but is instead learned fromthe trunk group ID and rules tag (RTAG) which is picked up from the ARLentry, and which can be used to identify the trunk port based upon thetrunk port index contained in the trunk group table. The RTAG and TGIDwhich are contained in the ARL entry therefore define which part of thepacket is used to generate the trunk port index. For example, if theRTAG value is 1, then the last three bits of the source MAC address areused to identify the trunk port index; using the trunk group table, thetrunk port index can then be used to identify the appropriate trunk portfor communication. If the RTAG value is 2, then it is the last threebits of the destination MAC address which are used to generate the trunkport index. If the RTAG is 3, then the last three bits of the source MACaddress are exclusive-Ored (XORED) with the last three bits of thedestination MAC address. The result of this operation is used togenerate the trunk port index. For IP packets, additional RTAG valuesare used so that the source IP and destination IP addresses are used forthe trunk port index, rather than the MAC addresses. SOC 10 isconfigured such that if a trunk port goes down or fails for any reason,notification is sent through CMIC 40 to CPU 52. CPU 52 is thenconfigured to automatically review the trunk group table, and VLANtables to make sure that the appropriate port bit maps are changed toreflect the fact that a port has gone down and is therefore removed.Similarly, when the trunk port or link is reestablished., the processhas to be reversed and a message must be sent to CPU 52 so that the VLANtables, trunk group tables, etc. can be updated to reflect the presenceof the trunk port.

Furthermore, it should be noted that since the trunk group is treated asa single logical link, the trunk group is configured to accept controlframes or control packets, also known as BPDUs, only one of the trunkports. The port based VLAN table, therefore, must be configured toreject incoming BPDUs of non-specified trunk ports. This rejection canbe easily set by the setting of a B bit in the VLAN table. IEEE standard802.1d defines an algorithm know n as the spanning tree algorithm, foravoiding data loops in switches where trunk groups exist. Referring toFIG. 19, a logical loop could exist between ports 1 and 2 and switchesSW1 and SW2. The spanning algorithm tree defines four separate states,with these states including disabling, blocking, listening, learning,and forwarding. The port based VLAN table is configured to enable CPU 52to program the ports for a specific ARL state, so that the ARL logictakes the appropriate action on the incoming packets. As notedpreviously, the B bit in the VLAN table provides the capability toreject BPDUs. The St bit in the ARL table enables the CPU to learn thestatic entries; as noted in FIG. 18, static entries are not aged by theaging process. The hit bit in the ARL table, as mentioned previously,enables the ARL engine 143 to detect whether or not there was a hit onthis entry. In other words, SOC 10 utilizes a unique configuration ofARL tables, VLAN tables, modules, etc. in order to provide an efficientsilicon based implementation of the spanning tree states.

In certain situations, such as a destination lookup failure (DLF)where apacket is sent to all ports on a VLAN, or a multicast packet, the trunkgroup bit map table is configured to pickup appropriate port informationso that the packet is not sent back to the members of the same sourcetrunk group. This prevents unnecessary traffic on the LAN, and maintainsthe efficiency at the trunk group.

IP/IPX

Referring again to FIG. 14, each EPIC 20 or GPIC 30 can be configured toenable support of both IP and IPX protocol at linespeed. Thisflexibility is provided without having any negative effect on systemperformance, and utilizes a table, implemented in silicon, which can beselected for IP protocol, IPX protocol, or a combination of IP protocoland IPX protocol. This capability is provided within logic circuitry1411, and utilizes an IP longest prefix cache lookup (IP_LPC), and anIPX longest prefix cache lookup (IPX_LPC). During the layer 3 lookup, anumber of concurrent searches are performed; an L3 fast lookup, and theIP longest prefix cache lookup, are concurrently performed if the packetis identified by the packet header as an IP packet. If the packet headeridentifies the packet as an IPX packet, the L3 fast lookup and the IPXlongest prefix cache lookup will be concurrently performed. It should benoted that ARL/L3 tables 21/31 include an IP default router table whichis utilized for an IP longest prefix cache lookup when the packet isidentified as an IP packet, and also includes an IPX default routertable which is utilized when the packet header identifies the packet asan IPX packet. Appropriate hexadecimal codes are used to determine thepacket types. If the packet is identified as neither an IP packet nor anIPX packet, the packet is directed to CPU 52 via CPS channel 80 and CMIC40. It should be noted that if the packet is identified as an IPXpacket, it could be any one of four types of IPX packets. The four typesare Ethernet 802.3, Ethernet 802.2, Ethernet SNAP, and Ethernet II.

The concurrent lookup of L3 and either IP or IPX are important to theperformance of SOC 10. In one embodiment of SOC 10, the L3 table wouldinclude a portion which has IP address information, and another portionwhich has IPX information, as the default router tables. These defaultrouter tables, as noted previously, are searched depending upon whetherthe packet is an IP packet or an IPX packet. In order to more clearlyillustrate the tables, the L3 table format for an L3 table within ARL/L3tables 21 is as follows:

IP or IPX Address—32 bits long—IP or IPX Address—is a 32 bit IP or IPXAddress. The Destination IP or IPX Address in a packet is used as a keyin searching this table.

Mac Address—48 bits long—Mac Address is really the next Hop Mac Address.This Mac address is used as the Destination Mac Address in the forwardedIP Packet.

Port Number—6 bits long—Port Number—is the port number the packet has togo out if the Destination IP Address matches this entry's IP Address.

L3 Interface Num—5 bits long—L3 Interface Num—This L3 Interface Numberis used to get the Router Mac Address from the L3 Interface Table.

L3 Hit Bit—1 bit long—L3 Hit bit—is used to check if there is hit onthis Entry. The hit bit is set when the Source IP Address search matchesthis entry. The L3 Aging Process ages the entry if this bit is not set.

Frame Type—2 bits long—Frame Type indicates type of IPX Frame (802.2,Ethernet II, SNAP and 802.3) accepted by this IPX Node. Value00—Ethernet II Frame. Value 01—SNAP Frame. Value 02—802.2 Frame. Value03—802.3 Frame.

Reserved—4 bits long—Reserved for future use.

The fields of the default IP router table are as follows:

IP Subnet Address—32 bits long—IP Subnet Address—is a 32 bit IP Addressof the Subnet.

Mac Address—48 bits long—Mac Address is really the next Hop Mac Addressand in this case is the Mac Address of the default Router.

Port Number—6 bits long—Port Number is the port number forwarded packethas to go out.

L3 Interface Num—5 bits long—L3 Interface Num is L3 Interface Number.

IP Subnet Bits—5 bits long—IP Subnet Bits is total number of Subnet Bitsin the Subnet Mask. These bits are ANDED with Destination IP Addressbefore comparing with Subnet Address.

C Bit—1 bit long—C Bit—If this bit is set then send the packet to CPUalso.

The fields of the default IPX router table within ARL/L3 tables 21 areas follows:

IPX Subnet Address—32 bits long—IPX Subnet Address is a 32 bit IPXAddress of the Subnet.

Mac Address—48 bits long—Mac Address is really the next Hop Mac Addressand in this case is the Mac Address of the default Router.

Port Number—6 bits long—Port Number is the port number forwarded packethas to go out.

L3 Interface Num—5 bits long—L3 Interface Num is L3 Interface Number.

IPX Subnet Bits—5 bits long—IPX Subnet Bits is total number of SubnetBits in the Subnet Mask. These bits are ANDED with Destination IPXAddress before comparing with Subnet Address.

C Bit—1 bit long—C Bit—If this bit is set then send the packet to CPUalso.

If a match is not found in the L3 table for the destination IP address,longest prefix match in the default IP router fails, then the packet isgiven to the CPU. Similarly, if a match is not found on the L3 table fora destination IPX address, and the longest prefix match in the defaultIPX router fails, then the packet is given to the CPU. The lookups aredone in parallel, but if the destination IP or IPX address is found inthe L3 table, then the results of the default router table lookup areabandoned.

The longest prefix cache lookup, whether it be for IP or IPX, includesrepetitive matching attempts of bits of the IP subnet address. Thelongest prefix match consists of ANDing the destination IP address withthe number of IP or IPX subnet bits and comparing the result with the IPsubnet address. Once a longest prefix match is found, as long as the TTLis not equal to one, then appropriate IP check sums are recalculated,the destination MAC address is replaced with the next hop MAC address,and the source MAC address is replaced with the router MAC address ofthe interface. The VLAN ID is obtained from the L3 interface table, andthe packet is then sent as either tagged or untagged, as appropriate. Ifthe C bit is set, a copy of the packet is sent to the CPU as may benecessary for learning or other CPU-related functions.

It should be noted, therefore, that if a packet arrives destined to aMAC address associated with a level 3 interface for a selected VLAN, theingress looks for a match at an IP/IPX destination subnet level. Ifthere is no IP/IPX destination subnet match, the packet is forwarded toCPU 52 for appropriate routing. However, if an IP/IPX match is made,then the MAC address of the next hop and the egress port number isidentified and the packet is appropriately forwarded.

In other words, the ingress of the EPIC 20 or GPIC 30 is configured withrespect to ARL/L3 tables 21 so that when a packet enters ingresssubmodule 14, the ingress can identify whether or not the packet is anIP packet or an IPX packet. IP packets are directed to an IP/ARL lookup,and IPX configured packets are directed to an IPX/ARL lookup. If an L3match is found during the L3 lookup, then the longest prefix matchlookups are abandoned.

HOL Blocking

SOC 10 incorporates some unique data flow characteristics, in ordermaximize efficiency and switching speed. In network communications, aconcept known as head-of-line or HOL blocking occurs when a port isattempting to send a packet to a congested port, and immediately behindthat packet is another packet which is intended to be sent to anun-congested port. The congestion at the destination port of the firstpacket would result in delay of the transfer of the second packet to theun-congested port. Each EPIC 20 and GPIC 30 within SOC 10 includes aunique HOL blocking mechanism in order to maximize throughput andminimize the negative effects that a single congested port would have ontraffic going to un-congested ports. For example, if a port on a GPIC30, with a data rate of, for example, 1000 megabits per second isattempting to send data to another port 24 a on EPIC 20 a, port 24 awould immediately be congested. Each port on each GPIC 30 and EPIC 20 isprogrammed by CPU 52 to have a high watermark and a low watermark perport per class of service (COS), with respect to buffer space within CBP50. The fact that the head of line blocking mechanism enables per portper COS head of line blocking prevention enables a more efficient dataflow than that which is known in the art. When the output queue for aparticular port hits the preprogrammed high watermark within theallocated buffer in CBP 50, PMMU 70 sends, on S channel 83, a COS queuestatus notification to the appropriate ingress module of the appropriateGPIC 30 or EPIC 20. When the message is received, the active portregister corresponding to the COS indicated in the message is updated.If the port bit for that particular port is set to zero, then theingress is configured to drop all packets going to that port. Althoughthe dropped packets will have a negative effect on communication to thecongested port, the dropping of the packets destined for congested portsenables packets going to un-congested ports to be expeditiouslyforwarded thereto. When the output queue goes below the preprogrammedlow watermark, PMMU 70 sends a COS queue status notification message onthe sideband channel with the bit set for the port. When the ingressgets this message, the bit corresponding to the port in the active portregister for the module can send the packet to the appropriate outputqueue. By waiting until the output queue goes below the low watermarkbefore re-activating the port, a hysteresis is built into the system toprevent constant activation and deactivation of the port based upon theforwarding of only one packet, or a small number of packets. It shouldbe noted that every module has an active port register. As an example,each COS per port may have four registers for storing the high watermarkand the low watermark; these registers can store data in terms of numberof cells on the output queue, or in terms of number of packets on theoutput queue. In the case of a unicast message, the packet is merelydropped; in the case of multicast or broadcast messages, the message isdropped with respect to congested ports, but forwarded to un-congestedports. PMMU 70 includes all logic required to implement this mechanismto prevent HOL blocking, with respect to budgeting of cells and packets.PMMU 70 includes an HOL blocking marker register to implement themechanism based upon cells. If the local cell count plus the global cellcount for a particular egress port exceeds the HOL blocking markerregister value, then PMMU 70 sends the HOL status notification message.PMMU 70 can also implement an early HOL notification, through the use ofa bit in the PMMU configuration register which is referred to as a UseAdvanced Warning Bit. If this bit is set, the PMMU 70 sends the HOLnotification message if the local cell count plus the global cell countplus 121 is greater than the value in the HOL blocking marker register.121 is the number of cells in a jumbo frame.

With respect to the hysteresis discussed above, it should be noted thatPMMU 70 implements both a spatial and a temporal hysteresis. When thelocal cell count plus global cell count value goes below the value inthe HOL blocking marker register, then a poaching timer value from aPMMU configuration register is used to load into a counter. The counteris decremented every 32 clock cycles. When the counter reaches 0, PMMU70 sends the HOL status message with the new port bit map. The bitcorresponding to the egress port is reset to 0, to indicate that thereis no more HOL blocking on the egress port. In order to carry on HOLblocking prevention based upon packets, a skid mark value is defined inthe PMMU configuration register. If the number of transaction queueentries plus the skid mark value is greater than the maximum transactionqueue size per COS, then PMMU 70 sends the COS queue status message onthe S channel. Once the ingress port receives this message, the ingressport will stop sending packets for this particular port and COScombination. Depending upon the configuration and the packet lengthreceived for the egress port, either the head of line blocking for thecell high watermark or the head of line blocking for the packet highwatermark may be reached first. This configuration, therefore, works toprevent either a small series of very large packets or a large series ofvery small packets from creating HOL blocking problems.

The low watermark discussed previously with respect to CBP admissionlogic is for the purpose of ensuring that independent of trafficconditions, each port will have appropriate buffer space allocated inthe CBP to prevent port starvation, and ensure that each port will beable to communicate with every other port to the extent that the networkcan support such communication.

Referring again to PMMU 70 illustrated in FIG. 10, CBM 71 is configuredto maximize availability of address pointers associated with incomingpackets from a free address pool. CBM 71, as noted previously, storesthe first cell pointer until incoming packet 112 is received andassembled either in CBP 50, or GBP 60. If the purge flag of thecorresponding P channel message is set, CBM 71 purges the incoming datapacket 112, and therefore makes the address pointers GPID/CPIDassociated with the incoming packet to be available. When the purge flagis set, therefore, CBM 71 essentially flushes or purges the packet fromprocessing of SOC 10, thereby preventing subsequent communication withthe associated egress manager 76 associated with the purged packet. CBM71 is also configured to communicate with egress managers 76 to deleteaged and congested packets. Aged and congested packets are directed toCBM 71 based upon the associated starting address pointer, and thereclaim unit within CBM 71 frees the pointers associated with thepackets to be deleted; this is, essentially, accomplished by modifyingthe free address pool to reflect this change. The memory budget value isupdated by decrementing the current value of the associated memory bythe number of data cells which are purged.

To summarize, resolved packets are placed on C channel 81 by ingresssubmodule 14 as discussed with respect to FIG. 8. CBM 71 interfaces withthe CPS channel, and every time there is a cell/packet addressed to anegress port, CBM 71 assigns cell pointers, and manages the linked list.A plurality of concurrent reassembly engines are provided, with onereassembly engine for each egress manager 76, and tracks the framestatus. Once a plurality of cells representing a packet is fully writteninto CBP 50, CBM 71 sends out CPIDs to the respective egress managers,as discussed above. The CPIDs point to the first cell of the packet inthe CBP; packet flow is then controlled by egress managers 76 totransaction MACs 140 once the CPID/GPID assignment is completed by CBM71. The budget register (not shown) of the respective egress manager 76is appropriately decremented by the number of cells associated with theegress, after the complete packet is written into the CBP 50. EGM 76writes the appropriate PIDs into its transaction FIFO. Since there aremultiple classes of service (COSs), then the egress manager 76 writesthe PIDs into the selected transaction FIFO corresponding to theselected COS. As will be discussed below with respect to FIG. 13, eachegress manager 76 has its own scheduler interfacing to the transactionpool or transaction FIFO on one side, and the packet pool or packet FIFOon the other side. The transaction FIFO includes all PIDs, and thepacket pool or packet FIFO includes only CPIDs. The packet FIFOinterfaces to the transaction FIFO, and initiates transmission basedupon requests from the transmission MAC. Once transmission is started,data is read from CBP 50 one cell at a time, based upon transaction FIFOrequests.

As noted previously, there is one egress manager for each port of everyEPIC 20 and GPIC 30, and is associated with egress sub-module 18. FIG.13 illustrates a block diagram of an egress manager 76 communicatingwith R channel 77. For each data packet 112 received by an ingresssubmodule 14 of an EPIC 20 of SOC 10, CBM 71 assigns a PointerIdentification (PID); if the packet 112 is admitted to CBP 50, the CBM71 assigns a CPID, and if the packet 112 is admitted to GBP 60, the CBM71 assigns a GPID number. At this time, CBM 71 notifies thecorresponding egress manager 76 which will handle the packet 112, andpasses the PID to the corresponding egress manager 76 through R channel77. In the case of a unicast packet, only one egress manager 76 wouldreceive the PID. However, if the incoming packet were a multicast orbroadcast packet, each egress manager 76 to which the packet is directedwill receive the PID. For this reason, a multicast or broadcast packetneeds only to be stored once in the appropriate memory, be it either CBP50 or GBP 60.

Each egress manager 76 includes an R channel interface unit (RCIF) 131,a transaction FIFO 132, a COS manager 133, a scheduler 134, anaccelerated packet flush unit (APF) 135, a memory read unit (MRU) 136, atime stamp check unit (TCU) 137, and an untag unit 138. MRU 136communicates with CMC 79, which is connected to CBP 50. Scheduler 134 isconnected to a packet FIFO 139. RCIF 131 handles all messages betweenCBM 71 and egress manager 76. When a packet 112 is received and storedin SOC 10, CBM 71 passes the packet information to RCIF 131 of theassociated egress manager 76. The packet information will include anindication of whether or not the packet is stored in CBP 50 or GBP 70,the size of the packet, and the PID. RCIF 131 then passes the receivedpacket information to transaction FIFO 132. Transaction FIFO 132 is afixed depth FIFO with eight COS priority queues, and is arranged as amatrix with a number of rows and columns. Each column of transactionFIFO 132 represents a class of service (COS), and the total number ofrows equals the number of transactions allowed for any one class ofservice. COS manager 133 works in conjunction with scheduler 134 inorder to provide policy based quality of service (QOS), based uponethernet standards. As data packets arrive in one or more of the COSpriority queues of transaction FIFO 132, scheduler 134 directs aselected packet pointer from one of the priority queues to the packetFIFO 139. The selection of the packet pointer is based upon a queuescheduling algorithm, which is programmed by a user through CPU 52,within COS manager 133. An example of a COS issue is video, whichrequires greater bandwidth than text documents. A data packet 112 ofvideo information may therefore be passed to packet FIFO 139 ahead of apacket associated with a text document. The COS manager 133 wouldtherefore direct scheduler 134 to select the packet pointer associatedwith the packet of video data.

The COS manager 133 can also be programmed using a strict priority basedscheduling method, or a weighted priority based scheduling method ofselecting the next packet pointer in transaction FIFO 132. Utilizing astrict priority based scheduling method, each of the eight COS priorityqueues are provided with a priority with respect to each other COSqueue. Any packets residing in the highest priority COS queue areextracted from transaction FIFO 132 for transmission. On the other hand,utilizing a weighted priority based scheduling scheme, each COS priorityqueue is provided with a programmable bandwidth. After assigning thequeue priority of each COS queue, each COS priority queue is given aminimum and a maximum bandwidth. The minimum and maximum bandwidthvalues are user programmable. Once the higher priority queues achievetheir minimum bandwidth value, COS manager 133 allocates any remainingbandwidth based upon any occurrence of exceeding the maximum bandwidthfor any one priority queue. This configuration guarantees that a maximumbandwidth will be achieved by the high priority queues, while the lowerpriority queues are provided with a lower bandwidth.

The programmable, nature of the COS manager enables the schedulingalgorithm to be modified based upon a user's specific needs. Forexample, COS manager 133 can consider a maximum packet delay value whichmust be met by a transaction FIFO queue. In other words, COS manager 133can require that a packet 112 is not delayed in transmission by themaximum packet delay value; this ensures that the data flow of highspeed data such as audio, video, and other real time data iscontinuously and smoothly transmitted.

If the requested packet is located in CBP 50, the CPID is passed fromtransaction FIFO 132 to packet FIFO 139. If the requested packet islocated in GBP 60, the scheduler initiates a fetch of the packet fromGBP 60 to CBP 50; packet FIFO 139 only utilizes valid CPID information,and does not utilize GPID information. The packet FIFO 139 onlycommunicates with the CBP and not the GBP. When the egress seeks toretrieve a packet, the packet can only be retrieved from the CBP; forthis reason, if the requested packet is located in the GBP 50, thescheduler fetches the packet so that the egress can properly retrievethe packet from the CBP.

APF 135 monitors the status of packet FIFO 139. After packet FIFO 139 isfull for a specified time period, APF 135 flushes out the packet FIFO.The CBM reclaim unit is provided with the packet pointers stored inpacket FIFO 139 by APF 135, and the reclaim unit is instructed by APF135 to release the packet pointers as part of the free address pool. APF135 also disables the ingress port 21 associated with the egress manager76.

While packet FIFO 139 receives the packet pointers from scheduler 134,MRU 136 extracts the packet pointers for dispatch to the proper egressport. After MRU 136 receives the packet pointer, it passes the packetpointer information to CMC 79, which retrieves each data cell from CBP50. MRU 136 passes the first data cell 112 a, incorporating cell headerinformation, to TCU 137 and untag unit 138. TCU 137 determines whetherthe packet has aged by comparing the time stamps stored within data cell112 a and the current time. If the storage time is greater than aprogrammable discard time, then packet 112 is discarded as an agedpacket. Additionally, if there is a pending request to untag the datacell 112 a, untag unit 138 will remove the tag header prior todispatching the packet. Tag headers are defined in IEEE Standard 802.1q.

Egress manager 76, through MRU 136, interfaces with transmission FIFO140, which is a transmission FIFO for an appropriate media accesscontroller (MAC); media access controllers are known in the ethernetart. MRU 136 prefetches the data packet 112 from the appropriate memory,and sends the packet to transmission FIFO 140, flagging the beginningand the ending of the packet. If necessary, transmission FIFO 140 willpad the packet so that the packet is 64 bytes in length.

As shown in FIG. 9, packet 112 is sliced or segmented into a pluralityof 64 byte data cells for handling within SOC 10. The segmentation ofpackets into cells simplifies handling thereof, and improvesgranularity, as well as making it simpler to adapt SOC 10 to cell-basedprotocols such as ATM. However, before the cells are transmitted out ofSOC 10, they must be reassembled into packet format for propercommunication in accordance with the appropriate communication protocol.A cell reassembly engine (not shown) is incorporated within each egressof SOC 10 to reassemble the sliced cells 112 a and 112 b into anappropriately processed and massaged packet for further communication.

FIG. 16 is a block diagram showing some of the elements of CPU interfaceor CMIC 40. In a preferred embodiment, CMIC 40 provides a 32 bit 66 MHzPCI interface, as well as an inter IC (I2C) interface between SOC 10 andexternal CPU 52. PCI communication is controlled by PCI core 41, and I2Ccommunication is performed by I2C core 42, through CMIC bus 167. Asshown in the figure, many CMIC 40 elements communicate with each otherthrough CMIC bus 167. The PCI interface is typically used forconfiguration and programming of SOC 10 elements such as rules tables,filter masks, packet handling, etc., as well as moving data to and fromthe CPU or other PCI uplink. The PCI interface is suitable for high endsystems wherein CPU 52 is a powerful CPU and running a sufficientprotocol stack as required to support layer two and layer threeswitching functions. The I2C interface is suitable for low end systems,where CPU 52 is primarily used for initialization. Low end systems wouldseldom change the configuration of SOC 10 after the switch is up andrunning.

CPU 52 is treated by SOC 10 as any other port. Therefore, CMIC 40 mustprovide necessary port functions much like other port functions definedabove. CMIC 40 supports all S channel commands and messages, therebyenabling CPU 52 to access the entire packet memory and register set;this also enables CPU 52 to issue insert and delete entries into ARL/L3tables, issue initialize cell free address pool (cFAP or CFAP)/slot freeaddress pool (sFAP or SFAP) commands, read/write memory commands andACKs, read/write register command and ACKs, etc. Internal to SOC 10,CMIC 40 interfaces to C channel 81, P channel 82, and S channel 83, andis capable of acting as an S channel master as well as S channel slave.To this end, CPU 52 must read or write 32-bit D words. For ARL tableinsertion and deletion, CMIC 40 supports buffering of four insert/deletemessages which can be polled or interrupt driven. ARL messages can alsobe placed directly into CPU memory through a DMA access using an ARL DMAcontroller 161. DMA controller 161 can interrupt CPU 52 after transferof any ARL message, or when all the requested ARL packets have beenplaced into CPU memory.

Communication between CMIC 40 and C channel 81/P channel 82 is performedthrough the use of CP-channel buffers 162 for buffering C and P channelmessages, and central processor (CP) bus interface 163. S channel ARLmessage buffers 164 and S channel bus interface 165 enable communicationwith S channel 83. As noted previously, PIO (Programmed Input/Output)registers are used, as illustrated by S-channel Programmed Input/Output(SCH PIO) registers 166 and PIO registers 168, to access the S channel,as well as to program other control, status, address, and dataregisters. PIO registers 168 communicate with CMIC bus 167 through I2Cslave interface 42 a and I2C master interface 42 b. DMA controller 161enables chaining, in memory, thereby allowing CPU 52 to transfermultiple packets of data without continuous CPU intervention. Each DMAchannel can therefore be programmed to perform a read or write DMAoperation. Specific descriptor formats may be selected as appropriate toexecute a desired DMA function according to application rules. Forreceiving cells from PMMU 70 for transfer to memory, if appropriate,CMIC 40 acts as an egress port, and follows egress protocol as discussedpreviously. For transferring cells to PMMU 70, CMIC 40 acts as aningress port, and follows ingress protocol as discussed previously. CMIC40 checks for active ports, COS queue availability and other ingressfunctions, as well as supporting the HOL blocking mechanism discussedabove. CMIC 40 supports single and burst PIO operations; however, burstshould be limited to S channel buffers and ARL insert/delete messagebuffers. Referring once again to I2C slave interface 42 a, the CMIC 40is configured to have an I2C slave address so that an external I2Cmaster can access registers of CMIC 40. CMIC 40 can inversely operate asan I2C master, and therefore, access other I2C slaves. It should benoted that CMIC 40 can also support media independent interfacemanagement (MIIM) through MIIM interface 169. MIIM support is defined byIEEE Standard 802.3u, and will not be further discussed herein.Similarly, other operational aspects of CMIC 40 are outside of the scopeof this invention.

A unique and advantageous aspect of SOC 10 is the ability of doingconcurrent lookups with respect to layer two (ARL), layer three, andfiltering. When an incoming packet comes in to an ingress submodule 14of either an EPIC 20 or a GPIC 30, as discussed previously, the moduleis capable of concurrently performing an address lookup to determine ifthe destination address is within a same VLAN as a source address; ifthe VLAN IDs are the same, layer 2 or ARL lookup should be sufficient toproperly switch the packet in a store and forward configuration. If theVLAN IDs are different, then layer three switching must occur based uponappropriate identification of the destination address, and switching toan appropriate port to get to the VLAN of the destination address. Layerthree switching, therefore, must be performed in order to cross VLANboundaries. Once SOC 10 determines that L3 switching is necessary, SOC10 identifies the MAC address of a destination router, based upon the L3lookup. L3 lookup is determined based upon a reading in the beginningportion of the packet of whether or not the L3 bit is set. If the L3 bitis set, then L3 lookup will be necessary in order to identifyappropriate routing instructions. If the lookup is unsuccessful, arequest is sent to CPU 52 and CPU 52 takes appropriate steps to identifyappropriate routing for the packet. Once the CPU has obtained theappropriate routing information, the information is stored in the L3lookup table, and for the next packet, the lookup will be successful andthe packet will be switched in the store and forward configuration.

Thus, the present invention comprises a method for allocating memorylocations of a network switch. The network switch has internal (on-chip)memory and an external (off-chip) memory. Memory locations are allocatedbetween the internal memory and the external memory according to apre-defined algorithm.

The pre-defined algorithm allocates memory locations between theinternal memory and the external memory based upon the amount ofinternal memory available for the egress port of the network switch fromwhich the data packet is to be transmitted by the network switch. Whenthe internal memory available for the egress port from which the datapacket is to be transmitted is above a predetermined threshold, then thedata packet is stored in the internal memory. When the internal memoryavailable for the egress port from which the data packet is to betransmitted is below the predetermined threshold value, then the datapacket is stored in the external memory.

Thus, this distributed hierarchical shared memory architecture defines aself-balancing mechanism. That is, for egress ports having few datapackets in their egress queues, the incoming data packets which are tobe switched to these egress ports are sent to the internal memory,whereas for egress ports having many data packets in their egressqueues, the incoming data packets which are to be switched to theseegress ports are stored in the external memory.

Preferably, any data packets which are stored in external memory aresubsequently re-routed back to the internal memory before being providedto an egress port for transmission from the network switch.

Thus, according to the present invention, the transmission line rate ismaintained on each egress port even though the architecture utilizesslower speed DRAMs for at least a portion of packet storage. Preferably,this distributed hierarchical shared memory architecture uses SRAM as apacket memory cache or internal memory and uses standard DRAMs or SDRAMsas an external memory, so as to provide a desired cost-benefit ratio.

Memory Management Unit.

The memory management unit, or PMMU 70, is generally configured tohandle all cell transactions from the time period in which an ingressfrom an EPIC 20 sends a cell out on the CPS channel 80 for storage inmemory, either internal 50 or external 60, until the time period inwhich an egress port receives the cell for transfer to a remote port.Further, PMMU 70 handles cell storage operations, both within CBP 50 andGBP 60, while isolating these memory storage operations from therespective ingress and egress ports, which serves to eliminate clocklatency problems between memory and the ports. Therefore, generallyspeaking, PMMU 70 conducts the following functions:

1) Receives cells from the ingress ports at the required rates;

2) Sends cells to the egress ports at the required rates;

3) Sends sideband messages to other modules to indicate the currentstatus and condition of PMMU 70;

4) Drops cells and/or packets in a manner that is not destructive to thechip, eg. causing ship seizure;

5) Controls and manages the storage of cells in CBP 50 and GBP 60; and

6) Supports debug read/write operations of internal registers andmemory.

In addition to the memory related functions, PMMU 70 as a wholeessentially operates to connect a plurality of sub-modules oroperational blocks together for operation as a common system known asPMMU 70. The sub-modules interconnected to form PMMU 70, for example,are generally shown in FIG. 20, and are as follows:

1) Cell Accumulation Unit 85 (CAU);

2) Status, Location, & Budget Manager 86 (SLBM);

3) Cell Free Address Pool 87 (cFAP);

4) CBP Controller 88;

5) Cell Accumulation Buffers 89 (CAB);

6) Cell Disassembly Unit 90 (CDU);

7) Slot Assembly Unit 91 (SAU);

8) Slot Disassembly Unit 92 (SDU);

9) Transaction Queue Controller 93 (XQ_ctrl);

10) Egress Scheduler 94 (ES);

11) Packet Pointer Pool Controller 95 (PPPC);

12) SDRAM Controller 96;

13) Slot Free Address Pool 97 (sFAP);

14) Cell Retrieval & Reclaim Unit 98 (CRRU); and

15) Slot Free Address Controller 99 (sFAP_ctrl).

MMU Interfacing With Other Modules

PMMU 70 uses the CP Bus 100, which is formed by C and P channels 81 & 82of CPS channel 80 for all cell-based traffic. As such, the CP Bus 100handles the bulk of the data transfer between PMMU 70 and other modulesin SOC 10. Ingress functions generally result in sending cells tomemory, either internal/on board memory (CBP 50) or external memory (GBP60), while the egress functions generally receive cells for transmissionto remote ports from PMMU 70. Therefore, these functions alone indicatethat PMMU 70 can generally require 50% of the available bandwidth of theCP bus 100. Further, since CP Bus 100 allows packets received into theingress to be split in to cells, transferred through the switch tomemory, and then reassembled into packets in memory, PMMU 70 is requiredto support reassembly of up to 32 packets at once given the exemplaryconfiguration noted above having 32 ports per switch.

CP Bus 100 supports variable length data transmissions, from 1-4 “words”per cell, wherein each word is 128 bits long. For example, a 65 bytepacket would come across CP Bus 100 as a 64 byte cell, in which all 4phases of P Bus 82 are valid. Thereafter, one word is transferred acrossthe bus, wherein only the first phase of P Bus 82 is valid. Thus, thedata on the first transfer, corresponding to when the S Bit is set,should be 4 words long, as when the S Bit is set, all of the data in thetrailing 3 phases is latched and used on subsequent cells sent by theingress. Therefore, PMMU 70 is configured to keep track of thefunctional data flow errors, such as 2 first cell indicators incurredwithout reading a last cell indicator therebetween. In this situation,PMMU 70 generates a memory failure message, which is sent out on Schannel 83 to all modules.

The S Channel 83, as mentioned above, is responsible for transferringsideband messages within SOC 10 and is the second interface between PMMU70 and other modules within SOC 10. In particular, S Channel 83 caries anumber of sideband messages between PMMU 70 and other modules within SOC10. As examples thereof, PMMU 70 is responsible for responding to thefollowing sideband messages:

1) Read Memory Command;

2) Write Memory Command;

3) Read Register Command;

4) Write Register Command;

5) Initialize CFAP;

6) Initialize SFAP;

7) Enter a Debug Mode; and

8) Exit a Debug Mode.

Further, PMMU 70 is responsible for generating and sending the followingsideband messages to other modules within SOC 10 on S Channel 83:

1) Back Pressure Warning Status;

2) Back Pressure Discard Status;

3) COS Queue Status;

4) Head of Line Status;

5) GBP Full Status;

6) GBP Available Notification;

7) Read Memory Ack;

8) Write Memory Ack;

9) Read Register Ack;

10) Write Register Ack; and

11) Memory Failure Notification.

The final interface between PMMU 70 and the remaining modules within SOC10 is an SDRAM interface. This interface, which is shown in FIG. 20 asSDRAM Controller 96, allows PMMU 70 to directly control an SDRAM bank ofmemory, which is illustrated as GBP 60 in the previously discussedexample configurations. In the present embodiment, for example, theSDRAM bank can be a 128-bit SDRAM bank, which can be at most 8 chips (16bits wide) and at most one physical bank, wherein all the data pins arearranged in a standard point to point SDRAM configuration. The exampleof the present embodiment supports SDRAM memory configurations from 16MB (8*16 MB chips) to 64 MB (8*64 MB chips). However, if x32 SDRAM's areavailable, then 4, 8, and 32 MB configurations would naturally beavailable, as 4*16 MB chips or 4*64 MB chips.

Timing

With the exception of a small portion of SDRAM Controller 96, PMMU 70 isclocked using the SOC 10 core clock frequency, which can be 133 MHz inthe example of the present embodiment. The SDRAM Clock is received intoPMMU 70 from an external oscillator, and sent out by PMMU 70 insynchronization with the data. This clock is generally specified tooperate in a range between 66 and 125 MHz, and has no relationship tothe main clock frequency of SOC 10. The specific functions and clockrelated features of SDRAM Controller 96 within the present example willbe further discussed herein.

In the example of the present embodiment, PMMU 70 accepts a new wordfrom an ingress port via CP Bus 100 every 4 clock cycless. Further, anew cell is sent to the egress ports every 4 clocks, again via CP Bus100. Exemplary cases for the maximum and minimum timing for theseoperations is illustrated in FIG. 21.

FIFO's in PMMU 70

Several First-In-First-Out queues (FIFO's) exist at the PMMUarchitecture level. Generally speaking, the FIFOs connect to the CBPread/write data bus and to the respective requestor. FIFOs includedwithin PMMU 70, for example, are an SLBM FIFO, an SDU FIFO, a CRRU CellFIFO, and a Debug/S-channel bus (SBUS) FIFO. Additionally, the randomaccess memories (RAMs) utilized by PMMU 70 are instantiated at thislevel. Examples of the respective RAM's in the present embodiment are asfollows:

1) CBP 50

2) SAU 91

3) SDU 92

4) CAB 89

5) Transaction Queues (XQ)

6) cFAP 87

7) Copy Count Pool (CCP)

8) sFAP 99

9) Packet Pointer Pool (PPP)

10) BC/MC Bitmap 7 Untagged (BC)

The architecture of PMMU 70 is configured to optimize timing andresources. Therefore, for example, the longest pipeline delay isgenerally no greater than 4 clock cycles. This 4 clock maximum pipelinedelay is best evidenced by referring to the 65 byte exemplary case shownin FIG. 21. As a result of the minimal pipeline delay specifications ofPMMU 70, the requirements for the above noted FIFOs are relatively high,as each stage must consume no more than 4 clock cycles of latency.

PMMU 70 further utilizes novel structure and logic within the respectiveFIFO's to optimally store data within storage units. This structure andlogic generally includes generating a glitchless fractional clock pulsefrom an increment or enable pulse and a clock signal, which is providedto a storage element to enable a data storage operation in a time periodin which the data to be stored is most stable. The glitchless fractionalclock pulse, which is generally of a shorter period than the system coreclock pulse and asserted high during the same time period or durationthat the core clock pulse is asserted high, defines a region in whichthe data to be stored is in an predicably stable state. The predictablystable state is a result of the data being stable in the median regionof the clock pulse, as opposed to the end regions proximate the risingand falling edge of the clock pulse, where the data tends to beunstable. The structure and logic of the present invention not onlyallows for storage of data during a predictably stable portion of aclock cycle, but also minimizes overhead consumption via usage of simplespace saving elements.

FIG. 31, for example, illustrates a structural configuration utilized togenerate the glitchless fractional clock pulse that is supplied to astorage unit 104. Although storage unit 104 is generally representedwithin FIG. 31 as a latch, it is contemplated within the scope of thepresent invention that other known storage devices and/or latchequivalents may be supplemented for latch 104. Storage unit 104 isprovided with two inputs: first, an enable or gate input thatcorresponds to the input where the previously discussed glitchlessfractional clock pulse is supplied; and a data input, where the data tobe stored in the latch is provided thereto. The data input, which isrepresented in FIG. 31 as Data_d, generally represents the data residenton a bus or similar structure awaiting storage within storage unit 104.Therefore, when the glitchless fractional clock pulse is presented atthe enable input of storage unit 104, the data resident at input D islatched or stored within storage unit 104. Therefore, it is desired tostrategically time the presentation of the glitchless fractional clockpulse to storage unit 104 to an optimal time when the data resident oninput D is most stable.

FIG. 32 shows an exemplary timing diagram of the memory circuitry shownin FIG. 31. CLK represents the general clock signal presented to thelogic circuitry within FIG. 31, and in particular, to the clock input ofelements 101 and 102. Data_d represents the data to be stored, and inparticular, Data_d is shown to have three specific blocks of data to bestored, which are labeled A, B, and C. The increment timing line (INC)represents a unique increment clock wide pulse supplied to element 101,which signals a bank of storage elements 104 to store/latch the dataresident on the Data_d line. WRPTR represents the write pointer.Incr(i), Incr(i+1), and Incr(i+2) represent the increment pulses withinvarious portions of the logic circuitry shown in FIG. 31. Finally,Gate(i) represents the glitchless fractional clock pulse generated atthe optimum time period in which to store/latch the data resident on theData_d line, as this data is most stable within the time period that theglitchless fractional clock pulse is asserted high. In particular, thispulse is generated in sync with the falling edge of the system clock,which places the clock pulse equidistant from the ends of the datawindow, as shown in FIG. 32.

Referring to the sample logic configuration shown in FIG. 31, a firstflip-flop 101 has an input (D) and a clock input (CLK). Input D receivesan increment pulse A and input CLK receives the system clock pulse B.Second flip-flop 102 also receives the system clock pulse on a CLKinput. However, the input D of second flip-flop 102 is connected to theoutput Q of first flip-flop 101. The output Q of first flip flop 101 isrepresented in FIG. 32 by the Incr(i])pulse. Output Q of first flip flop101 is also connected to a first input of an “AND” Gate 103. The secondinput of AND Gate 103, which is an inverted input, is connected to theoutput Q of second flip flop 102, which is represented in FIG. 32 by theIncr_n(i)pulse. This configuration generates the glitchless fractionalclock pulse denoted as Gate(i) in FIG. 32 from the output of AND Gate103. This glitchless fractional clock pulse is used as the activatinginput for latch 104, which operates to enable the latch in a time periodwithin the system clock pulse when the data resident on the Data_d lineis most stable. This stability affords much greater reliability andaccuracy in the data stored within latch/storage unit 104.

Although a particular logical arrangement has been disclosed in theexample shown in FIG. 32, it is understood by those in the art thatvarious equivalent logic arrangements can be created by De Morganizingthe configuration shown.

Reset and Initialization Events

PMMU 70 is configured to perform a standard built in self test (BIST)test on all internal memory structures upon a reset action of SOC 10.This test is conducted after the reset pulse diminishes. Subsequent tothe completion of the BIST test, cFAP 87 and sFAP 92 initialization willtake place. Assuming no BIST test problems, PMMU 70 is generally fullyoperational 64,000+n clocks after the reset pulse goes away, wherein “n”represents the number of clock cycles necessary to fully complete theBIST test. The 64,000 clock cycles represents an exemplary time periodrequired to initialize 64 MB of SDRAM, which would have 64,000 slots,each slot representing 1 k and one clock cycle for initialization.Further, in the present example, the initialization of the cFAP 87 takesplace in parallel to these operations, and generally requires only 4000clock cycles, which allows initialization time to be limited only tothat which is necessary for SDRAM to initialize.

During this initialization process, the arbiter for CP Bus 100 must beturned off, so that no requests on the S Bus 83 will be recognized andinterfere with the initialization process. Upon completion of memoryinitialization, the arbiter for the CP Bus 100 is turned back on, andthe default register settings loaded upon initialization are sufficientto allow for basic packet switching to begin. Any additional registersettings preferred by the user, such as predetermined watermarks ordiagnostic testing settings, can be modified after initialization.

Registers

The following read/write port specific (x32) registers are utilized byPMMU 70 in the present example.

1) Low Water Mark (LWM)—allocates the minimum number of cells that aport can consume within CBP 50.

2) High Water Mark (HWM)—allocates the maximum number of cells that aport can consume within CBP 50. The high water mark is essentially usedas a stopping point for bringing cells back in from GBP 60 into CBP 50for transmission to the appropriate egress port. In particular, if thelast cell count plus the global cell count is determined to be greaterthan the high water mark, then PMMU 70 is programmed to stop schedulingcells to be brought back in from GBP 60 until this congestion statusclears. Further, as an aside, every time a cell is brought in from GBP60, GCC is decremented and local cell count (LCC) in incremented.

3) Head of Line Blocking (HOL)—allocates the maximum number of cellsthat an egress can have outstanding at any given time.

4) Global Reroute Marker (GRM)—denotes the point at which PMMU 70 willstart to bring cells back into CBP 50 after these cells have gone out toGBP 60. This is specified in terms of cells remaining within GBP 60.

5) Ingress Back Pressure Warning Limit—denotes the number of cells atwhich PMMU 70 will send out a Back Pressure Warning message. Generally,in the present exemplary embodiment {15:0} corresponds to an examplewarning limit.

6) Scheduler Control—There are two fields that control scheduling flow.

a. Max Number of COS's—2 bits {1:0} to indicate the maximum number ofCOS's that the particular Egress will support. The logical depth of theXQ is then divided by the number of COS's.

b. Packet Pointer Pool Size—2 bits {17:16} used to indicate 16, 32, or64 entries in each PPP for each Egress.

PMMU 70 also utilizes COS specific (x8) registers. These registers areas follows.

Flow Control Registers—

a. Maximum Latency—{7:0} denotes the maximum time between packets(GPID/CPID's) being scheduled to go into the PPP.

b. Maximum Packets—{23:16} Denotes the maximum number of packets to sendon a particular COS. If the register is supplied with FF, then thisvalue denotes send until empty.

The present exemplary embodiment of PMMU 70 additionally utilizes anumber of general and/or multi-port registers (x1), which are asfollows.

1) CAB Bootout Timer—The number of clock cycles (in hundreds), beforecells are forced out from the CAB 89 (force a slot boundary).

2) CFAP Pool Size—The maximum number of cells to “dole out” for storagein the CBP 50.

3) SFAP Pool Size—The maximum number of cells to allocate for storage inthe GBP 60.

4) Sum of LWM's—The collective sum of all the Low Water Mark registers.This is used to pre-allocate space in CBP 50.

5) CBP Safety Margin—The number of cells to save for SDU 92 to bring into the CBP 50 (added to the Pre-allocation count) (prevents a deadlockcondition between CPID's and GPID retrieval).

6) XQ Skid mark—(8 bits) The number of entries to use as a safety marginfor the XQ filling. The SBUS message “COS Queue Status” will have anasserted bit for this COS if the SkidMark+the Preallocated Count+theOutstanding Transactions>=the total number of XQ entries (ex 256 if 8COS's are enabled). The Skid Mark is primarily used to cover the latencyof Ingress traffic responding to the S-bus message.

7) Misc. Configuration—Different bits for small functions

a) Disable Power management—Turns OFF any power management.

b) Short out unused Ports—Used by the Token Manager to “skip over” theports that have no instantiation.

c) Add dead state to SRAM's—Does not send a write request into an SRAMif a read is pending.

d) Start BIST—Setting this bit will start a BIST transaction.

Hardware will clear the bit when the BIST finite state machine (FSM) isdone. This is accomplished in DEBUG mode.

8) SDRAM Control—{3:0} bits denote the max number of cells per slot.{7:4} denote the maximum number of slots per chain (GPID). {15:8}control the DLL delay to the SDRAM CLKOUT0 {23:16} control the DLL delayto the SDRAM CLKOUT1 {31:24} control the DLL delay to the SDRAM CLKOUT2.

9) Clear Error Status—Writing this register will cause the correspondingbit(s) in the Global Error Register to be cleared.

10) Ingress Back Pressure Discard Delta Count—The number of cells atwhich PMMU 70 will send out a Back Pressure Discard message{15:0}=Discard Count that is added to the warning limit set in the Portspecific registers above.

PMMU 70 also includes a number of read only registers, which are listedbelow.

1) Local Cell Count (LCC_Count)—outstanding number of cells to an Egressthat resides in CBP 50, which can be (x32) port specific registers.

2) Global Cell Count (GCC_Count)—outstanding number of cells to anEgress that resides in the GBP 60, which can be (x32) port specificregisters.

3) XQ_entries—Total number of outstanding entries in the XQ, which canbe (x32) port specific register.

4) IBP_count—Current number of cells outstanding per Ingress, which canbe (x32) port specific register.

5) COS Specific (x8) registers.

6) General/MultiPort Registers (x1)

a. CFAP_readpointer—How many cells has PMMU 70 released

b. SFAP_readpointer—How many slots has PMMU 70 released

c. Dropped Cell Count—How many cells has PMMU 70 dropped due tobandwidth or error conditions.

d. Dropped Packet Count—How many packets has PMMU 70 dropped due tobandwidth or error conditions.

e. BIST Failed Address—bit 31 represents a failure having occurred,{30:n} represents which memory that failed, {n-1:0} represent theaddress that failed.

Cell Assembly Unit—CAU 85

The primary function of CAU 85 within the present exemplary embodimentof PMMU 70 is to convert data on CP Bus 100, which is in CP Bus format,into the Cell Data and Cell header formats used to store the Cell inmemory. This block also stores the code that is sent with the first 64bytes for each Ingress. Once a cell is received from CP bus 100 into CAU85, the cell is reformatted into 3 fields, the Cell Header, the CellData and the Sideband/info fields. Therefore, in the present example,CAU 85 is essentially a stand alone module with respect to its function,and as such, does not include any major sub-modules. However, CAU 85regularly interfaces with CP BUS 100, SLBM 86, and the system coreclock. In order for CAU 85 to operate within the desired parameters ofSOC 10, CAU 85 must process a cell every 4 clock cycles, if the cell isnot the first cell of a packet, and every 8 clock cycles if the cellhappens to be the first cell in a packet. CAU 85 utilizes a simple 4state finite state machine (FSM), that is configured to track CP Bus 100and its variable length. Thus, CAU 85 operates to pull cells off of CPBus 100 for processing by PMMU 70. Once a cell(s) is pulled off of CPBUS 100, CAU 85 formats the cell(s) into a selected commonformat/language utilized by the modules within PMMU 70. Upon completionof formatting the cell(s), CAU 85 hands the cell(s) off to SLBM 86.

Status, Location, & Budget Manager (SLBM 86)

The principle function of this module is to make decisions for cellsthat come into PMMU 70. Given the exemplary configuration having 32ports, cells can potentially come in at different times from differentingresses, and therefore, it is possible to have as many as 32 openpackets being reassembled at once within PMMU 70. Once a First Cell of apacket is received, all the necessary and relevant information forpacket storage is kept in a local RAM or register file. Correspondingly,each packet can be to one or many Egresses, as SOC 10, as noted above,has broadcast ad multicast capability. In the present example, eachEgress has 4 water marks associated therewith that signify the currentstatus of each outgoing port, which were discussed above. Further, SLBM86 interfaces to S-Bus 83 to communicate with other modules within PMMU70. SLBM 86 acts both to take register read-write commands and store theappropriate value corresponding thereto, and to send requests to stopingress traffic to a specific egress, or to possibly stop all incomingtraffic in a situation, for example, where GBP 60 is full.

An example of the data flow within SLBM 86 is shown in the flowchartpresented as FIG. 33. The general data flow within SLBM 86 begins withreceiving a cell at step 33-1 in the flowchart. At this point the logiccontinues to step 33-2, wherein it is determined if the FC bit, whichrepresents the first cell of the packet, is set. Since the FC bitindicates the first cell of a packet, the FC bit also serves to indicatewhether or not a block of data is currently being assembled within PMMU70, as this bit is set high upon entry of a cell into PMMU 70 andreleased or set low when the cell is processed into memory. If the FCbit is set, then the logic continues to step 33-3, wherein it isdetermined if the IP bit is set. If both the IP bit and the FC bits areset, then an error condition is determined at step 334, as a cellconstruction cannot be in progress when a first cell is received. If theIP bit is found not to be set, then the logic determines if the packetshould “go local” at step 33-5. The determination to store the incomingdata in local memory includes determining if the current cell count islower than the low watermark setting for the particular egress that thedata is scheduled to be transmitted to through SOC 10. Further, theGoLocal designation shown at step 33-5 additionally includes determiningif the GCC value is equal to zero, which corresponds to GBP 60 beingempty. Therefore, if the current LCC value plus the number of expectedcells left in this particular packet reassembly is less than the lowwatermark, then the packet is determined to be eligible for storage inlocal memory 50. If the packet is eligible, then the flowchart continuesto the start local accrual at step 33-6, which is further detailed inFIG. 34. If the logic determines not to go local, then the logiccontinues to the go global step, which is step 33-7. At step 33-7, thelogic determines the inverse of step 33-5, by determining if the localcell count, which is the number of cells in CBP 50, added to the numberof expected cells left in this particular reassembly is greater than thelow watermark. Alternatively, if the global cell count is determined tobe greater than zero, then the global accrual process is initiated. Ifthe determination is made to go global as a result of GCC being greaterthan 0 or the cell counts being over the LWM, then the logic continuesto the start global accrual in step 33-8, which is further detailed inFIG. 35. If the logic determines to not start the global accrual processat step 33-7, then it is next determined if the J bit is set at step33-9. If the J bit is set, then the logic determines if there issufficient memory space in global memory for a jumbo packet at step33-10. If sufficient space exists in global memory, the global accrualprocess is started; but if there is insufficient space, then the logiccontinues to step 33-11, wherein it is determined if there is room for amaximum size data block, which can, for example, correspond to up to 25cells. If so, the logic returns to the local accrual step, and if not,the logic checks to see if there is any room at all within local memoryfor the data block, so that local accrual may begin. If the J bit isdetermined not to be set at step 33-9, then the logic continues to step33-13 where the E bit is checked. The E bit corresponds to the end bit,so if it is set, the logic continues to step 33-12, and if it is not,then the logic continues to step 33-11.

Returning to step 33-2, if it is determined that the FC bit is not set,then the logic continues to step 33-14, wherein it is determined if theIP bit is set. If the IP bit is set, which corresponds to the statewherein the system is currently rerouting, then the logic continues to33-16 where it is determined if the system is currently rerouting. Ifthe system is currently rerouting, then the logic continues to step33-17, the continue global accrual step, which is further detailedwithin FIG. 37. If it is determined that the system is not currentlyrerouting, then the logic continues to step 33-18, which corresponds tothe continue local accrual step further detailed within FIG. 36.

The start local accrual process shown in FIG. 34 begins at step 34-1,where it is determined if the P bit is set. If the P bit is set, thenthe logic continues to sequential steps 34-2 and 34-3, where the dropcell count is incremented and the IP flag is cleared. Thereafter thelocal accrual process is completed at step 344. However, if the P bit isdetermined not to be set at step 34-1, then the logic continues with thelocal accrual process at steps 34-5 and 34-6, wherein the cell count isincremented, the first cell pointer is loaded, and the cell is writtento CBP 50. After the cell is written to CBP 50, the logic determineswhether the last cell (LC) bit is set at step 34-7. If the LC bit isdetermined to be set, then the logic writes the cell pointer into thetransaction queue at step 34-8 and continues to step 34-3. If the LC bitis determined not to be set, then the logic continues to steps 34-9 and34-10, where the IP flag is set and the next cell pointer is loaded.Further, the LPreallocation Count is calculated to be the sum of thecurrent LPreallocation Count and the {J, Max} cell count. Thereafter thestart local accrual process is completed at step 344, and the logicreturns to the receive cell step noted in FIG. 33.

The start global accrual process shown in FIG. 35 begins at step 35-1,where it is determined if the P bit is set. If the P bit is set, thenthe logic continues to sequential steps 35-2 and 35-3, where the dropcell count is incremented and the IP flag is cleared. Thereafter theglobal accrual process is completed at step 354. However, if the P bitis determined not to be set at step 35-1, then the logic continues withthe global accrual process at step 35-5, wherein it is determined if CAB89 is full or almost full. If CAB 89 is full or almost full, then thelogic continues to step 35-2. If not, then the logic continues to steps35-6 and 35-7, where the GCC is incremented and the cell is written toCAB 89. Subsequent to writing the cell to CAB 89, the logic determinesif the LC bit is set at step 35-8. If this bit is set, then the logicreturns to step 35-3. If the LC bit is not set, then the logic sets theIP flag and the GPreallocation Count is calculated to be the sum of thecurrent GPreallocation count and the {J, Max} cell count. Thereafter thelogic terminates at step 35-4 and returns to the receive cell stepwithin FIG. 33.

The continue local accrual process noted in FIG. 33 as step 33-18 iscontinued in FIG. 36 at step 36-1, where the logic determines if the Pbit is set. If it is determined that the P bit is set, then the logiccontinues through steps 36-2 through 36- 6. At step 36-2 the logic setsthe CellHeader P and LC bits along with incrementing thePurgedPacketCount (PPP). At step 36-3 the logic writes the cell to CBP50, and at step 364 the logic writes the FirstCellPointer (FCP) in thereclaim unit. At step 36-5 the logic clears the IP flag and at step 36-6the logic calculates the LPreallocationCount. Thereafter the logiccontinues to step 36-7, where the process is completed and the logicreturns to the receive cell step of FIG. 33. If the P bit is determinednot to be set at step 36-1, then the logic continues to steps 36-8 and36-9, wherein the LCC is incremented and the cell is written to CBP 50.Thereafter, at step 36-10 the logic determines if the LC bit is set. Ifthis bit is set, then the logic continues to step 36-12, wherein the FCPis written into the transaction queue. Thereafter, the logic continuesto steps 36-5 through 36-7. If the LC bit is determined not to be set atstep 36-10, then the logic loads the NextCellPointer (NCP) at step36-11, and then continues to step 36-7.

The continue global accrual process noted in FIG. 33 as step 33-17 isfurther detailed within FIG. 37. At step 37-1 in FIG. 37 the logicdetermines if the P bit is set. If it is determined that the P bit isset, then the logic continues to steps 37-2 -37through 37- 6, whereinthe CellHeader P and LC bits are set and the PurgedPacketCount isincremented at step 37-2. At step 37-3 the cell is written into CAB 89,and at step 37-4 the IP flag is cleared. At step 37-5 the logiccalculates the GPreallocationCount, and then continues to step 37-6,where the continue global accrual process is completed and the logicreturns to step 33-1 in FIG. 33. If the P bit is determined not to beset in step 37-1, then the logic continues to step 37-7, where it isdetermined if the CABFull bit is set. If this bit is set, then thepacket is purged at step 37-8, and the logic returns to step 33-1 inFIG. 33. If the CABFull bit is not set, then the logic determines if CAB89 is almost full and the status of the LC bit at step 37-9. If CAB 90is almost full and the LC bit is not set, then the logic continues tostep 37-2. If CAB 90 is not almost full and the LC bit is set, then thelogic continues to steps 37-10 and 37-11, wherein the GCC is incrementedand the cell is written into CAB 89. Thereafter, at step 37-12 the logicdetermines if the LC bit is set, and if so, the logic continues to step37-4 of the flowchart. If the LC bit is not set, then the logiccontinues to step 37-6 of the flowchart, which corresponds to returningto the receive cell step within FIG. 33.

In the example of the present embodiment, SBUS manager, which is asubmodule of SLBM 86, manages the SBUS interface (I/F) and disseminatesall necessary commands and information messages to the other internalmodules of SLBM 86 for relaying of status info and out of the PMMU 70.An Initial Decision Matrix submodule implements the flow configurationfor receiving a new cell within PMMU 70. The Initial Decision Matrixsubmodule ascertains whether the data is new or old, global, local, andabove or below the representative watermarks. The Initial DecisionMatrix submodule is an area between Ingress data, which, in the presentexample, is 32 bits wide, and Egress data, which is also 32 bits wide,and therefore a “matrix”. This submodule keeps track of the IP vectorand the First Packet Pointer (FPP) and Next Packet Pointer (NPP). TheInitial Decision Matrix submodule includes ingress cell pointers, whichkeep track of the current status of each packet assembly. There are 32registers corresponding to each ingress in the present example. Eachregister contains the FPP in the chain, the next cell in the chain andthe IP flag which states that the assembly is in progress. Additionally,the pointers and muxes that indicate to other modules that there is onlyone FPP, NPP, and IP flag are resident herein. The Initial DecisionMatrix submodule also interfaces to cFAP 87 by prefetching the necessarypointers. In the present example, the Initial Decision Matrix submoduleincludes a Sum32 submodule, which operates to sum a 32 bit input vectorand return an output. A Start Local Accrual and Start Global Accrualsubmodules are provided with pure combinatorial logic, and operate toimplement the cell flow logic discussed in the above noted flowchartsfor starting a new packet & subsequent cell accrual. A Continue LocalAccrual submodule is provided within the matrix. This module is alsopure combinatorial logic and implements the flow logic for continuing anold packet & any subsequent cell accrual. A Continue Global Accrualsubmodule is provided, again with pure combinatorial logic, and operatesto implement the cell flow logic for continuing an external (global)packet & subsequent cell accrual. A Cell Counter Unit submodule isprovided for keeping track of current cell counts for each egress. TheCell Counter Unit takes increment, decrement and transfer requests for aport that is being worked on (all can be at the same time) and keepstrack of the current local and global cell count for the respectiveegress. The Cell Counter Unit also generates status pins that notify theremaining modules within PMMU 70 of the current status. This module alsokeeps track of the Current Status of the module (reroute), and isinstantiated on a per-egress basis in the present example.

Cell Free Address Pool 87

Cell Free Address Pool 87, in the present example, is the module that isresponsible for obtaining and releasing free address from CBP 50 addresspool. The linked list architecture of CBP 50 mandates that the next cellheader be resident within PMMU 70 prior to the writing of the currentcell into memory. Therefore, every time a cell is written to CBP 50,another cell is fetched and becomes available to be written into thenext cell header for that ingress stream. This allows for accurategeneration and tracking of the linked list architecture. The primaryinterface with cFAP 87 is the Cell Free Address Pool Controller (cFAPctrl), which is shown in FIG. 20. The cFAP further interfaces with theread only cFAP_readpointer and the cFAP POOLSIZE, SUM OF LWM's, and CBPSafety Margin read/write registers.

Further, in the present exemplary embodiment, the configuration andoperational characteristics of cFAP 87 increase the reliability of SOC10, while decreasing the overhead usage. In particular, cFAP 87 and theaccompanying controller are configured to maximize the efficiency ofmemory usage through a unique address management scheme, which isillustrated by the following example. Furthermore, although this exampleof a memory management configuration is illustrated with respect to cFAP87, it can be effectively applied to various alternative memory systems.

For example, upon initialization of cFAP 87, BIST is conducted on thememory address locations within the memory unit corresponding to thememory address locations resident within cFAP 87. If the BIST finds anyerrors or defects in the memory structure tested, then the memoryaddress corresponding to the bad unit of memory is returned by the BIST.Thereafter, PMMU 70 removes this memory address location from the cFAP87, so that PMMU 70 does not access the bad memory unit during normaloperation. As such, the efficiency of SOC 10 is increased, as packetflow errors as a result of corrupted data being returned from bad memorylocation are minimized. Furthermore, replacement overhead is minimized,as a single BIST error will not render the memory inoperable, as theremaining usable memory locations are still utilized in the presentexemplary embodiment.

FIG. 39 is presented as further illustration and example of the memorymanagement configuration and method used within SOC 10. As noted above,cFAP 87 includes a plurality of memory addresses, wherein each memoryaddress corresponds to a physical memory location within the associatedmemory. Therefore, when a BIST indicates that a memory location withinthe memory associated with cFAP 87 is inoperable or unreliable, thenPMMU 70 essentially removes this faulty memory location from theavailable list of memory locations for use, which are contained in cFAP87. As shown in FIG. 39, when a bad memory location is found by a BIST,the address associated with the bad memory location is first locatedwithin cFAP 87. Thereafter, this memory address associated with theinoperable memory location is removed from the middle of the table ofavailable memory addresses and inserted at the top of the table ofavailable addresses in cFAP 87. Subsequent to the insertion of the badaddress into the top or first slot of the table of available memoryaddress within cFAP 87, the pointer used to indicate the first availablememory address is incremented to the next available memory address,which is immediately below the inoperable address just inserted withincFAP 87. Further, this pointer is initialized to return to thisposition, meaning the position below the inoperable address found by theBIST, when memory is empty and all addresses are available, so that thebad address is no longer accessed. Therefore, when a memory address isrequested from cFAP 87, the memory address corresponding to theinoperable memory location will no longer be used be used.

Additionally, cFAP 87 is configured to optimize the read and writeoperations of memory addresses from cFAP 871. In particular, forexample, cFAP 87 utilizes a last-in first-out configuration to saveoverhead and increase performance. This configuration is implementedthrough the use of a stack configuration to store cell free addresses,in conjunction with the stack pointer that indicates the next availableaddress for use from cFAP 87. As such, when an address is requested fromcFAP 87, the address that is utilized is the address that the stackpointer currently indicates as valid. Thereafter, the stack pointer isincremented into the stack to the next available address, which would bethe address adjacent to the address previously read from cFAP 87. Whenan address is released, the released address is placed at the top of thestack immediately below the stack pointer. Then the stack pointer isdecremented, which moves the pointer below the recently insertedaddress, to indicate that the address just received back into the stackis now the next available address for use. As such, a last-in first-outconfiguration is generated in the stack.

However, the present exemplary embodiment of the invention modifies thelast-in first-out configuration to further increase the performance ofthe memory, while also reducing the overhead requirements. Thismodification includes providing the capability to pass off an addressduring simultaneous read and write requests. In particular, when anaddress is released during the same clock cycle in which an address isrequested, then the present exemplary embodiment simply passes off thereleased address to the module requesting an address. As an example ofthis operation, assuming that a request for an address (a read request)was made during the same clock cycle as a release of an address (a writerequest), then the cFAP controller is configured to simply pass thereleased address to the module requesting an address. As such, there isno need to write the released address into the pool and decrement thepointer to indicate that the address is now available, which eliminatesmultiple clock cycles from the operation. Further, the present exemplaryembodiment eliminates the need to read an address from the address poolduring this process, as the released address is simply passed off to therequesting module without an address pool read operation and pointerincrement operation. The elimination of these steps via the passing offof the simultaneously released and requested address effectivelyeliminates the clock cycles associated with the aforementionedunnecessary accesses to memory. This elimination of clock cycles bothincreases the performance of the memory, as well as reduces the overheadnecessary to efficiently operate the memory.

Common Buffer Pool Controller (CBP_ctrl 88)

In the present exemplary embodiment, the Common Buffer Pool Controller88 (CBP_ctrl) module manages CBP 50. It takes requests from SLBM 86 forlocal storage within CBP 50 of cell requests from the read buffer fortransfer of cells, and schedules requests for the reading of cells to goout to an egress. CBP_ctrl 88 has the option of utilizing a first in,first out, (FIFO) operation to write requests to manage any latencybetween CBP_ctrl 88 and CP bus 100 transactions. CBP_ctrl 88 module isresponsible for the transfer of data to and from SDU 92, SLBM 86, andCRRU 98. Because of the tremendous bandwidth requirements, CBP_ctrl's 88sole purpose is to optimize CBP 50 bandwidth. CBP_ctrl 88 also dealswith any nuances (such as 2 clocks latency, dead cycle insertion etc.)to the CBP RAM itself, and other modules are not affected by theseactions.

CBP_ctrl 88 utilizes the core clock of SOC 10 and interfaces with twowrite FIFO's and one read FIFO. Further, CBP_ctrl 88 is configured tocontrol the random access memory of CBP 50. Depending upon the CBP 50random access memory specification, CBP_ctrl 88 has to fully optimizethe bandwidth of CBP 50. CBP_ctrl 88 is designed to interface to acommand FIFO (8 deep) and 3 external data FIFO's. Commands and data arepre-loaded, and the CBP_ctrl 88 strictly optimizes the transfer of data,working in any nuances, such as 1 clock dead cycle on W->R, if needed,or a 2 clock latency on data. This module is a cornerstone for theability PMMU 70 to accept writes from an ingress, SDU 92, and reads toCRRU 98 every clock cycle. CBP_ctrl 88 also includes three FSM's. First,an arbiter FSM that is responsible for placing requests into the commandqueue, in a prioritized fashion; second, an address FSM that isresponsible for the issuance of addresses and control signals to CBP 50in an optimal fashion; and third, a data FSM that is responsible for“moving data”, selecting mux's that go to the write data input, andasserting all the representative increment and decrement pulses.

Cell Disassembly Unit (CDU 90)

The primary function of the CDU 90 module in the present exemplaryembodiment is to convert the CBP Cell format back into the CP busformat. Therefore, CDU 90 generally accomplishes the inverse function ofCAU 85. CDU 90 takes the entire cell header and cell data, reformatsthese two elements as a combination, and sends the combination out on CPbus 100 in accordance with instructions from the arbiter. This is thereply phase of a transaction request by an egress. It is also the finalphase of the MMU data path, as the packet will be free of PMMU 70control upon reaching CP Bus format and being made available to CP Bus100 for transfer by the arbiter. CDU 90 also is responsible fordecrementing the ingress cell budget that affects the ingress backpressure warning/discard messages. In order to maintain the speed andbandwidth requirements of SOC 10, CDU 90 is configured to process 1 wordevery 4 clock cycles in the present exemplary embodiment.

Slot Assembly Unit (SAU 91)

The Slot Assembly Unit 91 (SAU) is responsible for pulling cells out ofCAB 89 and forming slots that will go to the SDRAM (external memory GBP60) as a large block. SAU 91, for example, has the ability to send 1-16cells per slot and will chain 1-16 slots per GPID. In order to maintainspeed and bandwidth requirements of SOC 10, SAU 91 pulls data from CAB89 every other clock cycle, and writes to SDRAM controller 96 everyother clock cycle. Therefore, for example, SAU 91 may read/pull data oneven clock cycles, while writing/pushing data on odd cycles, in order tomaximize the available bandwidth.

Slot Disassembly Unit (SDU 92)

SDU 92 is responsible for the inverse function of SAU 91, specificallythe reading of slots from 60 via the SDRAM Controller 96 and sendingthem to CBP 50. The SDU 92 parses through the slot and recognizes thebeginning and end of packets in the slots, and sends all CPID's to thePPP. SDU 92 in the present example is driven by a 32 deep command FIFO,when can be varied in size, that sets up the order in which SDU 92should read slots back from SDRAM/GBP 60. SDU 92 will keep track of the32 first slot pointers (FSP) and next slot pointers (NSP) for eachegress, and what position within the slot chain the process is currentlyoperating. SDU 92 operates on the core clock frequency of SOC 10, andwrites to CBP 50 every other clock cycle (on even clock cycles, forexample) and reads from CBP 50 on the alternative clock cycles (on oddclock cycles, for example).

Transaction Queue Controller (Xq_ctrl)

Transaction queue controller is configured to manage the transactionqueue for the specific egress that it represents. In the presentexample, XQ_ctrl takes requests to write a CPID or GPID to thetransaction queue and reads/writes the 2 k×18 XQ_RAM appropriately. TheTransaction queue controller is re-instantiated on a per-egress basis,and supports, for example, up to 8 COS levels. A feature of this “FIFO”is a “decrement->(then) read” policy. This is to prevent having to use 8registers per COS. Since only one entry in the transaction queue isworked on at a time, this is reasonable. Each transaction queue has 8COS's, and thus, the read/write pointers can point to 8 differentlocations. The transaction queue depth can be, for example, equal to 2 kentries divided by the number of COS's that it is supporting. Thus ifthe XQ_ctrl is supporting all 8 registers on a given egress, then itwill have 256 entries per COS. The BCBitmap that is sent to us with thefirst cell of a packet is used as a chip select for the transactionqueues. SLBM 86 and SAU 91 both write the transaction queues, however,for ordering issues, the writes of the respective modules are generallynot overlapping. If an egress port is currently rerouting, the writeoperations to that transaction queue will come from SAU 91, and not fromSLBM 86. Likewise SAU 91 will generally not write to a transaction queuethat is not currently rerouting.

The COSPtrCtrl module in the present example is responsible for the FIFOpointers, which eventually become addresses to an SRAM. It is staticallysized to adjust for tradeoff's between the maximum number of COS's andthe transaction queue depth. It also takes preallocation anddeallocation requests to give back a “stop accepting” notice and a“virtually full” notice to SLBM 86. The distinction between these twosignals is that the “stop accepting” is asserted when PMMU 70 is a“skidmark” transaction away from being “virtually” full. It is thende-asserted when PMMU 70 is 2 skidmarks away from being “virtuallyfull.” Virtually full is used by SLBM 86 to ascertain the status of thetransaction queue, thereby deciding whether or not to modify the BC/MCbitmap. If a transaction queue COS is full, SLBM 86 will mask off theBCBitmap bit that would otherwise send additional data to the alreadyfull transaction queue. This is a secondary mechanism for preventingdata overload at the transaction queue, as the ingress should stopsending more packets to this egress after the “COS Status” message goesout with that bit set. The consequences of that message not getting tothe ingress in time, or PMMU 70 not sending it in time can besignificant, as this situation could result in a lost “chain” in memory.The pre-allocate & de-allocate logic is used by CAB 89 to ensure that atransaction queue entry is “saved” for it. Once the GPID has beenwritten into the transaction queue by SAU 91, it can then de-allocateone entry. Writes to any single transaction queue can come at a maximumrate of one write instruction per 4 clock cycles However, there are 2sources for the transaction queue writes, SAU 91 and the SLBM 86. Thus,writes to non-overlapping transaction queues can occur back to back.Read operations occur independent of the write operations, and thus canoccur in the same clock cycle. In such a case, there will be a 1 clockdelay on the ReadAck signal back to Egress Scheduler 94.

Egress Scheduler 9

In the present exemplary embodiment, this module is responsible forpassing a “token” around in a token order to a scheduler for each egressmanager to take a request to move a packet from either GBP 60 or CBP 50into the PPP for that egress. For example if egress port 3 has a packetready to go, and the token comes around, egress port 3 makes a requestto the transaction queue to get the next packet for this port. Uponseeing the packet ID and the associated status of global or local,Egress Scheduler 94 then issues a write request to the PPP to take thatpointer, if it is a CPID, or hands a GPID off to SDU 92. If a GPID isbeing processed, it will issue a slot retrieval request every time atoken is passed to it. The use of a token/token order is for handlingthe transmission rate disparities among the respective egresses. Forexample, the Gigabit ports 30 will get the token 10× as often as the FEports 20, as the Gigabit ports operate to transfer data at 10 times therate of the Fast Ethernet ports. Further, Egress Scheduler 94 includestwo primary submodules. First, a COSArbiter, which is responsible fordeciding which COS gets serviced next within a given egress, and sendingRead Requests to the transaction queue. Within the COSArbiter is aCOSArbiter_fsm (x8), which is configured to manage Low Priority and HighPrioritay Requests, as well as determine when to decrement thetransaction queue. For example, a GPID request to SDU 92 involves manyReq/Ack transitions. High priority requests are used when the COSlatency timer has expired for this COS. Low priority requests will stayactive for a given COS until the maximum packet requirement has beenmet. The token manager determines the “rate” at which things getscheduled. If an Egress Scheduler 94 is not requesting, the token ispassed in the next clock. If an Egress Scheduler 94 is requesting, it is4 clocks to decrement, then transfer the packet to the PPP. The worstcase bandwidth requirement is the all egresses have minimum size packets(thus one CPID per packet) and thus one CPID per 8 clocks istransferred. Also within COSArbiter is a COSPriority Encoder (x1), whichis the priority encoder for the 8 levels of COS. This is a 16 bitencoder, wherein the top 16 are for high priority and the lower 16 arefor the low priority. This way a COS0 request that has had a latencytimeout, will get priority over a COS7 request that has not had alatency timeout. Second, COSArbiter includes an Egress Token Manager(x1), which operates to take in 32 tokens and hand out 32 tokens. Allegresses require a “token” in order to pass a CPID from the transactionqueue to the PPP, or to make a GPID or slot request to SDU 92. TheEgress Token Manager is designed to pass the token to the Gigabit ports10 times more often in order to maintain the desired flow within SOC 10.The token manager determines the “rate” at which data is scheduled toflow in or out of SOC 10. If an ES is not requesting, then the token ispassed to another scheduler in the next clock cycle. If an ES isrequesting, it takes 4 clock cycles to decrement the representativecount, and then transfer the packet to the PPP. In the worst casebandwidth requirement, all egresses have minimum size packets, andtherefore one CPID per packet. Therefore, one CPID should be transferredfor every 8 clock cycles. Further, Scheduler 94 includes 4 FSM's: first,a COSArbiter FSM; second, an Intercept FSM that intercepts tokens andproportionally distributes them to the Gigabit ports according to thedata transfer rate of the port; third, a Token Manager FSM, whichinitiates requests when Scheduler 94 has a token; and fourth, a GPIDManager, which interfaces with SDU 92 and requests slots and or GPID's.

Packet Pointer Pool Controller (PPP_ctrl)

PPP_ctrl's primary function is to manage the PPP RAM. PPP_ctrl takeswrite requests from Scheduler 94 and SDU 92 for storage of the FPP andalso read requests from CRRU 98 for taking the FPP. However, allrequestor's, namely the ESWrite31x0, SDUWrite31x0, & CRRURead31x0 in thepresent example, have at most one bit active at a time. Therefore, allrequests are treated independently, and subsequent requests willtypically be at most every 4 clocks. PPP_ctrl is also responsible forstoring all 32 read/write pointers for the PPP. Each pointer points tothe beginning of a packet linked list. SDU 92 and Scheduler 94 blockswill write at an appropriate rate corresponding to the rate at which theacknowledge signals return, and likewise CRRU 98 will read at anequivalent speed. PPP_ctrl will have shallow FIFO's around it to absorbany transient bursts in requests from other modules. However, in orderto meet the speed and bandwidth requirements of SOC 10, PPP_ctrl isconfigured to service a read request every 8 clock cycles and 2 writerequests every 8 clock cycles, all of which are independent events.However, the PPP_ctrl can theoretically operate as fast as theacknowledge signals are received, and therefore, performance of SOC 10is generally not affected by PPP_ctrl's operation. Further, PPP_ctrlincludes three FSM's: first, an Arbiter_FSM that is responsible forplacing requests into the Command Queue; second, an Address_FSM that isresponsible for the issuance of addresses and control signals to CBP 50;and third, a Data_FSM that is responsible for moving data, selectingmultiplexers that go to the wr_data input, and asserting all the correctIncrement and decrement pulses.

SDRAM Scheduler 96

The SDRAM Scheduler 96 in the present exemplary embodiment isresponsible for taking requests from a number of sources, for example,SAU 91, SDU 92, SFAP 97, and the Refresh requestor. These requests areprocessed and sent out to the SDRAM. As such, SDRAM Scheduler 96 mustinterface between SAU 91, SDU 92, and SFAP 97, which is accomplishedthrough a synchronous interface. The function of SDRAM Scheduler 96 isto arbitrate and schedule requests from these three units. Althoughorder is preserved in processing requests from SAU 91, SDU 92, and SFAP97, there is some degree of flexibility in the logic controlling SDRAMScheduler 96 to select which request from the aforementioned units isprocessed next. More particularly, the logic governing the operation ofSDRAM scheduler 96 attempts to minimize and/or optimize the overheadrequirements of the module by considering predetermined factors inaccessing the memory. Examples of these could factors include, switchingbetween read and write accesses, wherein the switching occurs within ina four-clock overhead per read/write pair. Another example would be tominimize when a row miss occurs in the same bank, which results inexcessive overhead usage, generally in the range of approximately fiveclock cycles plus read/write overhead. Further, the logic attempts tomaximize row hits, as row hits utilize the least overhead. A portion ofthis optimization process is similar to logical speculation on the partof SDRAM Controller 96, as for any slot read operation, it may benecessary to write back an updated copy count, and the logic will notknow this until after the read has actually been initiated.

In addition to considering overhead factors, SDRAM Controller 96considers the priority designation of the requests and/or the associateddata in accessing SDRAM. For example, if all of the requests from SAU 96get filled, then SDRAM Controller 96 can put a higher priority onwriting to SDRAM, which will mitigate the possibility of droppingpackets as a result of not enough write operations to SDRAM. The SDUgoing empty is less of a priority, as generally no packets are droppedin this case. SFAP 97 is generally implemented as a stack. If at anytime, SFAP 97 needs to both read and write the SDRAM at once, then thepointers will be copied over internally, which renders SFAP similar to asingle-level command register.

In operation in the present example, SDRAM Controller 96 functions as anarbiter for read and write operations to global memory 60. However,SDRAM Controller 96 must process requests for access to GBP 60 from anumber of sources, as noted above. Therefore, in order to maximize theefficiency of each memory access, SDRAM Controller 96 essentiallypre-plans its accesses. In particular, SDRAM Controller 96 reviews thecurrent requests and determines which of the requests can be processedtogether in order to minimize overhead. For example, if Controller 96 isreceiving simultaneous requests from SAU 91, SDU 92, SFAP 97, and theRefresh requestor, then Controller 96 looks at the first requestreceived and determines the clock overhead cost associated withprocessing this first request alone. Then Controller 96 looks at each ofthe remaining requests and determines the overhead cost associated withprocessing each individual request in conjunction with the first requestreceived. Thereafter, Controller 96 selects the request that can be mostefficiently paired with the first request received, and processes thisrequest in conjunction with the first request received. As such, tworequests are processed within the overhead generally associated with asingle memory access, as the second request is essentially processedwithin the shadow of the first request for purposes of clock overhead.Further, Controller 96 is capable of utilizing this method to group aplurality of requests together using the same method for processingunder a single clock overhead, which obviously increases the efficiencyof Controller 96.

With regard to timing issues, SFAP 97 will activate its request signalduring the same clock that SFAP 97 makes a command valid, as shown inFIG. 22. Eventually, the SDRAM Controller 96 will reply with an Acksignal. This may be a few hundred clock cycles, if Controller 96 isdealing with a priority situation with the SAU's or other modules. Whilewaiting for the Ack signal, the SFAP 97 can continue to access itsinternal SRAM. Once the Ack signal is returned to SFAP 97, it needs toswitch off its internal SRAM accesses. In a minimum of two clocks afterAck goes active, Xfr will go active initiating a read or write transferof eight words with the SFAP 97 internal SRAM. Due to clock speeddifferences between the SOC 10 and SDRAM, there may be one or more idleclock cycles during this transfer. During any idle clocks, transfer willbe inactive, as Ack remains active.

In the present example, SAU 91 write requests and SDU 92 read requestswill be queued in a six-deep FIFO. This configuration allows one commandfor each half of each SAU 91 and SDU 92 present. The format for SAU 91and SDU 92 numbers, one of three is selected for the transfer. 00selects “A”, 01 selects “B” and 10 selects “C”. The timing of these SAU91 and SDU 92 commands to THE SDRAM Scheduler is fairly simple. Thereare enough command entries in the FIFOs to allow one command to bequeued for each half of each SAU/SDU, so there is no need for a “full”signal to indicate the FIFO is full. The Sau2SdramReq and Sdu2SdramReqsignals go active to indicate a command is ready on the Sau2SdramCmd orSdu2SdramCmd bus respectively.

The SDRAM Controller 96 addresses SAU 91 and SDU 92 blocks using a 9-bitaddress bus in the present exemplary embodiment. This bus selects whichSAU/SDU unit (A, B or C) as well as the one of 24 312-bit words withinthe SAU/SDU, then one of three 128-bit subwords within the word. Thesame refer to the “SAU to SDRAM” and “SDRAM to SDU” timing diagrams,which are shown as FIGS. 23 and 24 respectively. When reading from theSAU, Sdram2SauAddr is driven valid and Sdram2SauRd goes active to read a128-bit subword on the Sau2SdramData bus. This data is available oneclock after Sdram2SauAddr becomes valid. During the same clock that thelast sub-word is addressed, Sdram2SauLast will go active to inform theSAU logic this buffer is again available for filling.

When writing to SDU 92, Sdram2SduAddr is valid during the same clockcycle that a valid sub-word is present on Sdram2SduData. Between one andthree sub-words can be written to any SDU word, depending in the celllength. The SDRAM scheduler provides two signals to control this latchand write function, so that read-modify-write logic in the SDU can beavoided. Sdram2SduLd goes active with each sub-word, then Sdram2SduWrgoes active on the last sub-word, indicating that the assembled completeword should be written to SDU 92. A last signal (Sdram2SduLast) is alsoprovided to indicate the final data transfer. Referring specifically toFIGS. 23 and 24, it is shown, for example, that the “C” word beingwritten will first contain the side-band data bits (sub-word 2),followed by cell data bytes 15-0 (sub-word 0) then cell data bytes 31-16(sub-word 1). Sdram2SduWr goes active during the C1 sub-word to requestthe SDU logic to write the completed word.

The interface between Scheduler 94 and SDRAM Controller 96 isasynchronous. Commands are passed in a four-entry asynchronous FIFO.Each data transfer takes at least two SDRAM words, as the minimum sizedcell in a one-cell slot would consist of one word of side-band data, andone word containing cell data. With complete two-clock synchronizationboth directions, it takes six clock cycles of the internal clock of SOC10 for one stage of an asynchronous FIFO to turn around. Therefore,three FIFO entries are generally used for the command FIFO. A fourthcommand FIFO entry is used to facilitate pipelining the top of FIFOcommand with the one already in progress in the SDRAM controller. TheSDRAM clock will be asynchronous to SOC10's internal clock, and runningat a lower frequency.

There are separate FIFOs for SDRAM writes and reads, each of which arefour levels deep. However, if a full handshake protocol were needed onboth ends, six levels would likely be necessary. However, there is nofull handshake needed on the SDRAM controller 96 side, as the data ispre-fetched and the associated memory space is pre-allocated.

The SDRAM Scheduler needs to keep a record of read and write commands soit can match up the FIFO data with its source or destination. Read andwrite transfers happen at different times, so there is generally aseparate command tracker for reads and writes. Each tracker is sixentries, as this is enough to cover all four command FIFO entries, plusthe current command in progress at SDRAM Controller 96, with one extraentry remaining. As such, the possibility of a tracker overflowing isminimized. The information maintained consists, for example, of only twobits. The SDRAM scheduler combines commands from the three sources notedabove into a single command format for SDRAM Controller 96. Part of thescheduler's function is determining “same” or “different” bank, and row“hit” or row “miss.” Therefore, SDRAM scheduler must know the SDRAMsorganization and be able to split row and column addresses.

The timing of SDRAM Controller 96 is illustrated in FIGS. 25, 26, and27. FIG. 26 illustrates the timing of writing commands to SDRAMController 96. When a command is ready to be written, SdramCmdReq isactivated, and the SdramCmd bus is driven. Then the logic checks to seeif SdramCmdFull is active. If it is, the logic holds Req active and Cmdvalid while waiting for another clock cycle in which the Full indicatoris inactive. If the command FIFO is not full at first, then command “A”is written, which fills the FIFO. One clock cycle later, the FIFO is notfull, and command “B” is written. Command “B” fills the FIFO again, butthere are no new commands ready so Req is driven inactive.

Therefore, the timing for writing data into the Data Write FIFO issimilar to the processes associated with writing the command FIFO, whichare noted above. However, the processes associated with reading datafrom the Data Read FIFO is different from the aforementioned processes.When reading data, SdramDRdReq is activated, while waiting forSdramDRdEmpty to be inactive, as shown in FIG. 27. During any clockcycle that Req is active, and Empty is inactive, the SdramDRd bus willcontain valid data read from the SDRAM. Using this timing scheme, dataword “A” is available immediately upon Req going active. Words “B” and“C” are available after a one clock cycle dead time.

A cell can occupy one or two words on the bus between SLBM 86 and CAB89, depending on the length of the particular cells, and the bus is 312bits wide in the present example. If the length is 25 bytes or less,generally one word is enough, otherwise a second word is needed. Thefirst word contains all side-band information, in case the second wordis missing. FIG. 28 illustrates the first and second word formats.

With regard to the bus between SAU 91 and CAB 89, as data is taken outof CAB 89, the BC/MC Bitmap, Untagged Bitmap, and COS fields are removedand are copied into the transaction queue. The data bytes in SAU 91 arearranged differently, to more closely match the SDRAM format. The firstSAU word of a cell contains data for the first three SDRAM words, andthe second SAU word, if present, contains data for the fourth and fifthSDRAM words. There are 32-bits of sideband data for each cell in thepresent example. According to the logic of CAB slot filling, all cellsin a slot are going to the same destination egresses. This means thatthe copy count on all cells in a slot will theoretically be identical.For this reason, PMMU 70 can safely put a common copy count in a uniformplace in the first word of each slot, and thereafter, simply use thecommon copy count for the first cell and any subsequent cells. However,the SDRAM interface of the present invention compresses slot data totake the least amount of 128-bit SDRAM words. A two bit “Cell Size”field in the 32 bits of sideband data indicates how many words the cellwill take. FIG. 29 illustrates number of words within SAU 91 and SDRAMthat correspond to four possible two bit cell sizes. The second wordonly contains bytes 63-32 of cell data, and the “copy count” field isonly valid for the first cell in a slot, as shown in the SAU word formatillustrated in FIG. 30.

In the bus between SAU 91 and SDRAM, subsequent words each contain up to16 bytes of cell data, if present. The slot size, which is defined as aprecise count of the number of SDRAM words used in this slot field, isadded as the slot is written to SDRAM.

When multiplexing data for SDRAM, attention is needed to make sure thecell data bytes are sent in the correct order. Specifically, forexample, the sideband bytes are sent first (coming from bits 256-292 ofthe first SAU word), followed by data bytes 15-0 (bits 127-0 of the sameword), then data bytes 31-16 (bits 255-128). SDRAM to SDU bus/SDUformat. The bus is 128 bits wide, per the SDRAM interface, and timemultiplexed. Data will be latched inside SDU 92 to put together 312-bitwords. The bus is basically in SDRAM format, except the next cell header(NC Header) field replaces Slot Size and Copy Count. A “Release” flag isadded to inform the SDU logic that this slot should be released back tothe free-slot pointer pool. This is generally set for slots notcontaining multicast or broadcast cells.

In SDU 92 to CBP 50 bus, which is generally in CBP format, the cell databytes are split differently. This bus is 288 bits, of which 28 bytes ofcell data are in the first word, and the remaining 36 bytes are in thesecond word. This means four bytes of data from the first SDU word willend up in the second CBP word, but this situation occurs only ininstances where cells longer than 28 bytes are present on the SDU 92 toCBP 50 bus.

Packet Flow Within the MMU

Upon completion of the initialization process, assuming that there wereno memory failure errors encountered in the BIST test, then PMMU 70 isready to receive cells. In the present exemplary embodiment, the firstcells received from any ingress are generally 4 words long, whichcorresponds to 512 bits, and all 4 phases of the P Bus should be valid.PMMU 70 has the ability to recognize disparities in cell flow, such as a“no last cell” indicator, and further, PMMU 70 tests for theseconditions as a form of maintaining data integrity. Once PMMU 70 beginsto receive cells, CAU 85 immediately translates the data coming in offof the CP Bus 100. This translation returns the Cell Header, Cell Data,and the Cell Sideband Busses. As the cell is being received, the sourceand destination fields (BCBitmap) are used by the next stage in thepipeline. The next stage in the pipeline, as shown in FIG. 20, is SLBM86. SLBM 86 looks at budget registers, cFAP status, CAB status, SDRAMstatus, and the transastion queue depth to determine the destination ofthe packet traveling through PMMU 70. A packets destination can only beevaluated upon completed reception of the first cell, as thisinformation is encapsulated within the first cell. After the destinationis determined, this destination must be retained, as the entire group ofassociated or linked cells forming the assembled packet must be eithersaved in CBP 50, GBP 60, or the entire group of linked or associatedcells must be dropped. This is to avoid mishandling of cells anddisordering of cell assembly, as retrieving packet information frommultiple memory resources is undesirable. The CurrentlyRerouting Flagsin SLBM 86, which are also termed the in progress (IP) flags, are setupon the reception of each first cell within PMMU 70, and are referredto for each remaining cell in the packet. This operation is furtherdiscussed in the SLBM section above. Further, upon reception of thefirst cell, PMMU 70 allocates the appropriate memory space within CBP50, if this space is available. Then it must be determined whether thecells/packet is to be stored locally within CBP 50 or globally in GBP60. This determination is made by comparing a predetermined threshold,termed herein a Low Water Mark, with a calculated value. In order todetermine if a cell is eligible for entry into CBP 50, the current cellcount must be lower than the Low Water Mark, and the Global Cell Countmust equal 0. If these two conditions are met in the present exemplaryembodiment, then the packet or cells can be stored locally. If not, thenthe packet/cells must be stored in global memory 60. Once SLBM 86 hasdetermined the storage destination of the packet and made the decisionas to which memory address to begin storing the packet in, then SLBM 86takes the first cell of the packet and stores this cell in CBP 50, ifeach cell corresponding to the currently transferred packet has beenselected for storage within internal memory. If the determination ismade to send each of the cells to external memory, as a result of awatermark within internal memory being exceeded, for example, then thefirst cell and all cells associated therewith are sent to CAB 89 forprocessing into GBP 60 by SDRAM controller 96. In the case of broadcastor multicast packets, the cell is sent to both, with the BCBitmap andthe copycount fields being adjusted to reflect this operation.

If the packet is determined to be eligible for admission into CBP 50,then it has passed through the SLBM flowcharts, which are furtherdiscussed herein above, and is written in to the SLBM cell FIFO thatgoes into CBP 50. However, before writing the current cell into CBP 50,SLBM 86 is required to pre-fetch the next cell pointer (NCP), such thatthe following cells can be tied or linked to the first cell of thepacket. This essentially allows for storage of multiple cells of asingle packet into numerous memory locations, while also allowing for aneffective and accurate mechanism for reassembling the cells into asingle packet identical packet to that which was sliced and stored inmemory. For each packet reassembly, SLBM 86 stores a first packetpointer (FPP) and an NCP. As such, upon reassembly, the reassemblingmodule knows where the first cell of the packet is located, as well aswhere the next cell of the packet is located. Thus, using thesepointers, the respective reassembly module can “string together” aplurality of linked cells into a single packet. These pointers arestored for each port on SOC 10, and therefore, in the present exemplaryembodiment there are 32 pointers to be stored for each SOC 10. Eachadditional cell received by SLBM 86 requires the fetching of another NCPfrom cFAP 87. This process of retrieving and storing cell pointers willcontinue for each port until PMMU 70 receives the last cell. Once thelast cell in the packet is received, PMMU 70 writes the cell to CBP 50with a void value in the NCP field. Further PMMU 70 writes the FPP tothe transaction queue controller, with the G/L flag set to local. Thus,PMMU 70 uses the stored BCBitmap that was stored when the first cell wasreceived to “chip select” the various transaction queues and write theUT bits in to each queue. Further, the transaction queue controller isrequired to know which COS queue this is going to, which is valid in thefirst phase of P Bus 81.

CBP controller 88 is responsible for maximizing the throughput of cellsto and from CBP 50. Therefore, CBP controller 88 also requiressubstantial FIFO's. In the present example, CBP controller 88 has an 8deep command queue that decides SLBM 86 write requests, SDU writerequests, and CRRU write requests. These requests can be any length,generally 1-4 words, and stacked into each data FIFO, independent of thecommand that requests the service. For example, SLBM 86 can insert a 4word data unit into an SLBM receive cell data FIFO, while the requestfor service is conducted in parallel. The command queue and the dataqueue are de-coupled to allow for greater bandwidth efficiency, and assuch, SOC 10 of the present embodiment is capable of a true 17+ Gb/sbandwidth utilization our of CBP 50, which is sufficient to keep aheadof the CP Bus 100 bandwidth.

If it is determined that a packet should be sent to external memory,which corresponds to GBP 60, then the first cell is sent from SLBM 86 toCAB 89. At this point, when an external memory storage operation isinitiated, SLBM 86 no longer has any responsibility for the presentpacket, and only notes that the packet is being rerouted to GBP 60. Oncethe first cell has reached CAB 89, it is stored until enough cells arepresent to form a “slot.” CAB 89 stores cells according to theirrespective ingress, but “packages” them in accordance with thecommonality of the egress port, COS, and the UT tags of the respectivepackets. The rules for these classifications are further discussed inthe CAB section above. However, once enough cells have been collected inCAB 89 to form a slot, then CAB 89 sends a request to SAU 91 to transferthe slot of cells into SDRAM/GBP 60. The present configuration of PMMU70, for example, generally utilizes 2 CAB's, 3 SAU's, and 3 SDU's forbandwidth and handling the variable CP word length. The SAU's, theSDU's, and the CAB's have a 2× bandwidth requirement, as PMMU 70 mustread and write for every cell received, which also justifies the wordlength of 312 bits.

After SAU 91 has receive the request from CAB 89, SAU 91 performs thesame prefetch an store operations as previously mentioned for SLBM 86relative to FPP and NCP, such that the same “linked list” of cells iscreated. Specifically, SAU 91 has to store the first slot pointer (FSP),the next slot pointer (NSP), and a status it as to whether or not theentire slot chain (GPID) has been written into GBP 60. However, in orderto prevent an out of ordering state, the GPID cannot be written to thetransaction queue controller until the entire slot chain has beenwritten to GBP 60. In this regard the SFAP_Controller 99 performs muchthe same function that the cFAP_controller did. However, SFAP_Controller99 has but a small portion of the SDRAM/GBP 60 slots cached in the localSFAP memory 97. Consequently, the write request by SAU 91 has to contendwith 3 other requestor's for the SDRAM_Controller's 96 attention, whichare the refresh request, the SFAP swap requests, and the SDU readrequest. In a tight bandwidth situation, PMMU 70 will favor writerequests from the SAU 91 over the read requests from the SDU92. Once theentire chain of cells forming a linked list representing the originalsliced packet to be stored is written into GBP 60, the GPID is writteninto the transaction queue with the appropriate COS.

With the entry in the transaction queue at a particular COS, PMMU 70must “schedule” the cells associated therewith for transmission to thedesignated egress port(s). The transaction queues can be logicallydivided into as many COS queue's as desired, on a per port basis. Forexample, port 3 may support 4 COS's, which makes the “logical depth” ofthe COS queue=2 k/4=512 entries/COS. Likewise, port 5 may have 8 levels,in which 256 entries/COS may be stored. Each ID in the transactionqueue, can be a pointer to the external memory/CBP 60 chain thatcontains 1 to (16*12) cells, or a pointer to the internal memory, thatis already resident. The egress scheduler receives and reviews all ofthe outstanding requests for each COS. There are 32 egress schedulers inthe present example, each one being chartered with scheduling the nextpacket to be sent to the PPP. The determination of which packet of theseveral that may be resident in each COS queue next to be scheduled is afunction of 2 variables, MaxLatency, and MaxPackets. These 2 variablesare COS specific (i.e. applicable to generally all egresses) anddetermine the maximum time that a COS entry is allowed to wait, beforeit gets a higher request priority, and exactly how many packets we cansend from this COS queue, before allowing alternative packets to bescheduled. Assuming that a packet is ready to be sent to the appropriateegress, it is still necessary to arbitrate with the other egresstransaction queue for scheduling the Packet ID to be moved directly tothe PPP, if the packet resides in CBP 50, or schedule for retrieval fromthe SDRAM, if the packet is stored in external memory GBP 60.

The Egress Token Manager (ETM) is the module that passes a token aroundto each of the 32 Egress schedulers, offering the opportunity for theindividual scheduler to move a Packet ID from the transaction queue (iflocal) into the PPP, or to request a slot retrieval from SDU 92. Two ofthe ports are Gigabit ports, and in an effort to minimize buffering, theGigabit ports receive the token 10× as often as the Fast Ethernet ports.Once the appropriate egress scheduler has the token, the output of thetransaction queue is directly connected to the PPP or the SDU 92. In theevent of a local ID, it is written directly into the PPP. In the eventthat it is a global ID, SDU 92 is now charged with retrieving the GPID,one slot at a time.

Once the token is passed, the egress scheduler and the SDU 92 engage inan interaction. The ES makes a GPIDRequest, to which the SDU 92 willeventually respond with a GPIDAck, or a SLOTAck. In the event of aGPIDAck, the Slot that came back was the last in the chain (which mayalso be the first). In the event that a SLOTAck is returned, the SDUstores the NSP. When the ES gets the token again, it makes a SLOTReq, inlieu of a GPIDReq, and the interaction between the SDU and the EScontinues (with SLOTReq and SLOTAck) until SDU 92 has reached the end ofthe chain and returns a GPIDAck. At this point the ES can “retire” thetransaction queue entry and move to it's next Packet ID for transferalto the PPP. The SDU 92 receives GPID requests from 32 egress schedulers.It has a command queue depth of 32, which corresponds to one queue foreach ES. It takes in GPID requests and writes the FSP into a table,indexed by the port ID. At the other end of the queue, read requests arebeing made by SDU 92 to retrieve slots from the SDRAM Controller 96.Once the data is returned, the SDU parses through the Slot and breaksthe data into cells. SDU 92 stores the NSP, if this is not the last Slotin the chain, and then transfers these cells back into the CBP 50through the SDU cell buffer, and writes any complete CPID's into thePPP. If it is the last slot in the chain, SDU 92 will return a GPIDAck.If it is not the last slot in the chain, the SDU 92 will return aSLOTAck to indicate that there are more slots in the chain and that theES should return with a SLOTReq the next time it gets the Token. It isin this fashion, that the SDU is able to minimize the latency that anyEgress will incur in receiving packets.

The PPP is divided into 32 logical partitions, each having a physicalmaximum of 64 entries. Each port has a programmable option to make thelogical maximum number of entries to be 8, 16, 32, or 64, based on a 2bit input (per port). The next to last stage in this pipeline ofoperations is the Cell Retrieval and Reclaim Unit 98. This module isresponsible for looking at the PPP empty flag and scheduling cellretrievals from CBP 50. The Egress has to pull cells from the PMMU 70,so this is managed through a simple single bit I/F wire that pulses highonce per clock edge for each cell that it wants. Again, a token managermonitors all the egress cell counters, validates that there are entriesin the PPP pool, and verifies that the token manager is able to acceptthe entries. With the token at this procedural point, a read request issent to CBP 50.

With the cell scheduled for reading, concurrently, is the read access tothe Copy Count Pool. If the value in the pool is equal to one, then thecell is transferred back to the cFAP controller to return the addressback for reclamation. At this point the process is rejuvenated. Once thecell is sent to the CRRU Cell FIFO, CDU 90 will pull the data out of theCRRU Cell FIFO and convert the Cell format back into the CP bus format.The CRRU will also “pick off” the LC bits and the NCP in order to keeptrack of where in the cell chain each egress is at. Once the CDU has thecell, it is sent to the egress and the MMU has completed its operationrelative to that particular cell.

Error Detection

As noted above, PMMU 70 operates to detect packet transfer and memoryerrors. When an error condition is detected, or alternatively, when theIngress tells us to purge the current packet, PMMU 70 handles theoperations associated with purging the packet and returning the systemto normal operation. Assuming that a purge packet request generated bythe ingress was received after the majority of the packet has beenstored in CBP 50 or GBP 60, PMMU 70 prematurely terminates the packet.This is accomplished by setting the LC and P bits in the header. If theentire packet is already residing in local memory, then the FPP is sentto the CRRU block, where it is stored in a reclaim queue. If the packetis resident in external memory, then the cell will go out to the SDRAMwith the LC and P bits set, and the GPID will still be assigned to thetransaction queues. However, when SDU 92 brings the slot(s) back in, itwill recognize the P bit has been set in the cell header and instead ofputting the FPP into the PPP, it will send it to CRRU 98. Using thisconfiguration and method of operation, there exists a small, but finitechance that the reclaim queue can become over-congested, at which pointPMMU 70 must stop all Ingress traffic. This is evidenced by a STOPrequest to the CP arbiter that will stop granting Ingress Requests. PMMU70 is configured to prioritize reclaim requests over egress requests.Under these circumstances, the egresses and the ingresses couldunder-run and overrun respectively. The ingress should apply its ownback pressure mechanism if it detects an overflow condition, in order toprevent a perpetual, positive feedback condition. This condition isexacerbated when the ingress overflows, it has to terminate the packet,causing more reclaim requests, causing more STOP requests to thearbiter.

In these situations, PMMU 70 will detect/generate parity (checksum) on aper slot basis. As the slot goes into SAU 91, a checksum is generated inthe slot header. A checksum is used to allow modification for the copycount manipulation. As the slot is pulled out of the SDRAM/GBP 60, SDU92 does a check on the data & checksum. If there is an error, the CPU isnotified via the S-Bus message “Memory Failure Notification” and it isthen under software control to handle the error. A CPU response might beto communicate to the Egresses to simply flush all of the packets itreceives for a predetermined time period, which would correspond to themaximum time required for flushing a transaction queue.

During packet processing, PMMU 70 is also configured to implementtemporal and spatially based flow control within SOC 10. For example, ifa particular egress port is receiving an excessive number of cells fortransmission, then PMMU 70 is configured to implement a combination oftemporal and spatial flow control methods to remedy the overcrowding atthat particular egress port. This overcrowding situation generallyoccurs when a Gigabit port is acting as an ingress for packets destinedfor egress on a normal Ethernet port, as the Gigabit port can fill anegress queue at a rate 10 to 100 times faster than normal Ethernet portscan clear and transmit packets from the queue. Another overcrowdingsituation occurs when a number of normal Ethernet ports are receivingpackets from a plurality of ports, but all of the packets received fromthe Ethernet ports are destined for a single egress port. As such, theegress queue again fills at a much faster rate than the egress port canclear and transmit packets from the queue.

In order to prevent port congestion in these instances, a flow controlscheme must be implemented to temporarily “switch off” data flow to aport that has or is becoming congested. Therefore, the first taskassociated with implementing a flow control scheme is to continuallytrack and/or count the number of packets or cells destined for each porton SOC 10. Further, a first and second predetermined thresholds must beestablished for each port, wherein the first threshold 110 representsthe point at which data flow should be shut off in order to avoid portcongestion and the second threshold 111 represents a marker point forreinitiating data flow to the port upon leaving a congestion state.These thresholds are illustrated in FIG. 38. Thus, when an incoming dataflow (cells residing in queue for egress) reaches the firstpredetermined threshold 110, noted as “A” in FIG. 38, then the incomingdata flow is discontinued for that port, as the port is approaching thecongestion state illustrated in FIG. 38. Subsequently, since theoutgoing data flow at the particular egress is still operational, thenumber of cells residing in queue for this particular egress willdecrease, and eventually fall below the second predetermined threshold111, which is noted as “B” in FIG. 38. When the number of cells in queuefor egress falls below second threshold 111 at point “B,” then theegress port has essentially returned to the normal operational range,which is illustrated in FIG. 38. After the data flow re-enters thenormal range at point “B,” PMMU 70 waits to reinitiate data flow to theegress for a predetermined amount of time “τ,” as illustrated in FIG.38. Therefore, upon re-entering the normal range and waiting apredetermined amount of time “τ,” data flow is reinitiated at point “C.”As such, PMMU 70 has utilized a unique temporal and spatial flow controltechnique, which results in an overhead savings, space savings, gatesavings, and reduces the number of dropped packets within a networkswitch as a result of port congestion.

The above-discussed configuration of the invention is, in a preferredembodiment, embodied on a semiconductor substrate, such as silicon, withappropriate semiconductor manufacturing techniques and based upon acircuit layout which would, based upon the embodiments discussed above,be apparent to those skilled in the art. A person of skill in the artwith respect to semiconductor design and manufacturing would be able toimplement the various modules, interfaces, and tables, buffers, etc. ofthe present invention onto a single semiconductor substrate, based uponthe architectural description discussed above. It would also be withinthe scope of the invention to implement the disclosed elements of theinvention in discrete electronic components, thereby taking advantage ofthe functional aspects of the invention without maximizing theadvantages through the use of a single semiconductor substrate.

Although the invention has been described based upon these preferredembodiments, it would be apparent to those of skilled in the art thatcertain modifications, variations, and alternative constructions wouldbe apparent, while remaining within the spirit and scope of theinvention. In order to determine the metes and bounds of the invention,therefore, reference should be made to the appended claims.

What is claimed is:
 1. A method for optimizing access to memory, saidmethod comprising the steps of: receiving a first request for access toa memory; receiving at least two additional requests for access to thememory; determining a first clock overhead associated with the firstrequest for access to the memory; determining an additional clockoverhead associated with each of the at least two additional requestsfor access to the memory in conjunction with the first request;determining a combination of requests that can be processed togetherusing an optimized overhead; and processing the combination of requestsas a single request with the optimized overhead.
 2. A method foroptimizing access to memory as recited in claim 1, wherein saidoptimized overhead further comprises a combination of said first clockoverhead and said additional clock overhead.
 3. A method for optimizingaccess to SDRAM in a network switch, said method comprising the stepsof: receiving a plurality of requests for access to an SDRAM; andcombining at least two of the plurality of requests for processing as asingle request utilizing an optimized clock overhead, in accordance witha predetermined algorithm.
 4. A method for optimizing access to SDRAM ina network switch as recited in claim 3, wherein the combining stepfurther comprises the steps of: determining a necessary clock overheadfor a first request for access to SDRAM; determining a necessary clockoverhead for the remaining plurality of requests for access to SDRAM;determining an optimal request from the remaining plurality of requests,wherein the optimal request is calculated to generate the optimized dockoverhead when combined with the first request for access to SDRAM; andprocessing the first request for access to the SDRAM simultaneously withthe optimal request.
 5. A method for optimizing access to SDRAM asrecited in claim 4, wherein the step of determining an optimal requestfurther comprises the steps of: determining an overhead associated withindividually processing each of the remaining plurality of requests incombination with the first request; and determining which combination ofrequests uses the least clock overhead.
 6. A method for optimizing SDRAMin a network switch, said method comprising the steps of: receiving afirst request for access to an SDRAM; receiving a second, third, andfourth request for access to the SDRAM; determining the clock overheadassociated with processing the first request in conjunction with thesecond request; determining a clock overhead associated with processingthe first request in conjunction with the third request; determining adock overhead associated with processing the first request inconjunction with the fourth request; determining an optimal request foraccess to SDRAM, said optimal request being calculated to yield aminimal clock overhead; and processing the optimal request.
 7. Anapparatus for optimizing access to memory in a network switch, saidapparatus comprising: means for receiving a first request for access toa memory; means for receiving at least two additional requests foraccess to the memory; means for determining a first dock overheadassociated with the first request for access to the memory; means fordetermining an additional clock overhead associated with each of the atleast two additional requests for access to the memory in conjunctionwith the first request; means for determining a combination of requeststhat can be processed together using an optimized overhead; and meansfor processing the combination of requests as a single request with theoptimal overhead.
 8. An apparatus for optimizing access to memory in anetwork switch as recited in claim 7, wherein said means for a firstrequest for access to a memory and said means for receiving at least twoadditional requests further comprises a memory controller.
 9. Anapparatus for optimizing access to memory in a network switch as recitedin claim 8, wherein said memory controller further comprises an SDRAMcontroller.
 10. An apparatus for optimizing access to memory in anetwork switch as recited in claim 7, wherein said means for determininga first dock overhead and said means for determining an additional clockoverhead further comprises a memory controller.
 11. An apparatus foroptimizing access to memory in a network switch as recited in claim 10,wherein said memory controller further comprises an SDRAM controller.12. An apparatus for optimizing access to memory in a network switch asrecited in claim 7, wherein said means for determining a combination ofrequests and said means for processing the combination of requestsfurther comprises a memory controller.
 13. An apparatus for optimizingaccess to memory in a network switch as recited in claim 12, whereinsaid memory controller further comprises an SDRAM controller.